order of exceptions in array_to_datetime
First, cleans things up by moving all the seen_datetime and seen_integer variables to the tops of their respective blocks.

Mainly splits the `try:` block for `string_to_dts` up into two pieces based on the two lines in that block that _could_ raise and handles errors more specfically.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19621?src=pr&el=h1) Report
> Merging [#19621](https://codecov.io/gh/pandas-dev/pandas/pull/19621?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a214915e241ea15f3d072d54930d0e0c8f42ee10?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19621/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19621?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19621      +/-   ##
==========================================
+ Coverage   91.59%    91.6%   +<.01%     
==========================================
  Files         150      150              
  Lines       48795    48798       +3     
==========================================
+ Hits        44696    44699       +3     
  Misses       4099     4099
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.97% <ø> (ø)` | :arrow_up: |
| #single | `41.73% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19621?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/19621/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2Jhc2UucHk=) | `60.6% <0%> (+3.93%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19621?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/19621?src=pr&el=footer). Last update [a214915...f70792b](https://codecov.io/gh/pandas-dev/pandas/pull/19621?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks. ideally want to split up these giant functions (e.g. make smaller functions to handle different types, like the strings) so it easier to read. but progress.