BUG: Raise KeyError when indexing a Series with MultiIndex
- [x] closes #14885
- [x] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @ryanreh99! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:







There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-04-24 14:32:03 UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26155?src=pr&el=h1) Report
> Merging [#26155](https://codecov.io/gh/pandas-dev/pandas/pull/26155?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c79b7bb91236a2e4cde3782d23109ac1f5e76dce?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26155/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26155?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26155      +/-   ##
==========================================
- Coverage      92%   91.98%   -0.02%     
==========================================
  Files         175      175              
  Lines       52371    52378       +7     
==========================================
- Hits        48184    48182       -2     
- Misses       4187     4196       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.54% <100%> (-0.01%)` | :arrow_down: |
| #single | `40.74% <0%> (-0.15%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26155?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26155/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `90.56% <100%> (-0.32%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26155/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `75% <0%> (-12.5%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26155/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.9% <0%> (-0.12%)` | :arrow_down: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/26155/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.94% <0%> (-0.06%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26155?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26155?src=pr&el=footer). Last update [c79b7bb...31df19b](https://codecov.io/gh/pandas-dev/pandas/pull/26155?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26155?src=pr&el=h1) Report
> Merging [#26155](https://codecov.io/gh/pandas-dev/pandas/pull/26155?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/947bd7662afa8b6dc4406ec52cefc39d76d2d6cd?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26155/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26155?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26155      +/-   ##
==========================================
- Coverage   91.98%   91.97%   -0.02%     
==========================================
  Files         175      175              
  Lines       52371    52375       +4     
==========================================
- Hits        48175    48171       -4     
- Misses       4196     4204       +8
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.52% <100%> (-0.01%)` | :arrow_down: |
| #single | `40.7% <0%> (-0.16%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26155?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26155/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `90.53% <100%> (-0.35%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26155/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/computation/engines.py](https://codecov.io/gh/pandas-dev/pandas/pull/26155/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vZW5naW5lcy5weQ==) | `88.33% <0%> (-0.2%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26155/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.9% <0%> (-0.12%)` | :arrow_down: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/26155/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.94% <0%> (-0.06%)` | :arrow_down: |
| [pandas/core/algorithms.py](https://codecov.io/gh/pandas-dev/pandas/pull/26155/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWxnb3JpdGhtcy5weQ==) | `94.8% <0%> (+0.01%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26155/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.71% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26155?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26155?src=pr&el=footer). Last update [947bd76...2309e14](https://codecov.io/gh/pandas-dev/pandas/pull/26155?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 hello @jreback ping made a change to the if statement as a `KeyError` wasn't raised for a particular condition. Added that test. thanks @ryanreh99 very nice  @jreback thanks for all the help :D 