array_to_datetime set seen_datetime consistently
From tslib.array_to_datetime

```
                try:
                    check_dts_bounds(&dts)
                    seen_datetime = 1
                except ValueError:
                    if is_coerce:
                        iresult[i] = NPY_NAT
                        continue
                    raise
```

If this raises, it gets caught and tries a different strategy, that strategy depending on `seen_datetime`.  This pattern shows up twice, and in each case, a date/datetime _has_ been seen regardless of whether `check_dts_bound`s raises.

Is there a reason why we're not setting `seen_datetime=1` unconditionally before the bounds check?