cleanup order of operations kludges
DatetimeIndexOpsMixin has an unfortunate kludge where it checks if `self` is a `TimedeltaIndex` and if so dispatches to a reversed operation.  This then makes subsequent checks less straightforward than they could/should be.  This fixes that.

This is in preparation for separating out DatetimeArray/TimedeltaArray/PeriodArray, so this uses formatting on error messages to be class-agnostic.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19895?src=pr&el=h1) Report
> Merging [#19895](https://codecov.io/gh/pandas-dev/pandas/pull/19895?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8f1dfa74e244622a7f356a496072f93dce032839?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `91.3%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19895/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19895?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19895      +/-   ##
==========================================
- Coverage   91.67%   91.66%   -0.01%     
==========================================
  Files         150      150              
  Lines       48936    48930       -6     
==========================================
- Hits        44860    44853       -7     
- Misses       4076     4077       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.04% <91.3%> (-0.01%)` | :arrow_down: |
| #single | `41.82% <34.78%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19895?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/19895/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `91.21% <100%> (+0.21%)` | :arrow_up: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/19895/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.61% <75%> (ø)` | :arrow_up: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/19895/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `97.33% <88.88%> (-0.06%)` | :arrow_down: |
| [pandas/core/dtypes/cast.py](https://codecov.io/gh/pandas-dev/pandas/pull/19895/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2Nhc3QucHk=) | `87.68% <0%> (-0.3%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19895?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/19895?src=pr&el=footer). Last update [8f1dfa7...e34c4a5](https://codecov.io/gh/pandas-dev/pandas/pull/19895?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!