BUG: raise KeyError if MultiIndex.get_loc_level is asked unused label
- [x] closes #22221
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22230?src=pr&el=h1) Report
> Merging [#22230](https://codecov.io/gh/pandas-dev/pandas/pull/22230?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d52748c3ef40505b6aec2e9b850d616fe17a81fd?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22230/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/22230?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22230      +/-   ##
==========================================
+ Coverage   92.07%   92.07%   +<.01%     
==========================================
  Files         169      169              
  Lines       50683    50684       +1     
==========================================
+ Hits        46666    46668       +2     
+ Misses       4017     4016       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.48% <100%> (ø)` | :arrow_up: |
| #single | `42.34% <10%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22230?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/22230/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `95.33% <100%> (+0.08%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22230?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/22230?src=pr&el=footer). Last update [d52748c...31b55c4](https://codecov.io/gh/pandas-dev/pandas/pull/22230?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 can you also add the original OP test somewhere. thanks @toobaz 