sunsch validates configuration on run

ert recently added functionality to [validate forward model steps on startup](https://ert.readthedocs.io/en/latest/getting_started/howto/plugin_system.html#forward-models). `SUNSCH` accounts for a large amount of forward model failures in FMU. If an error can be detected when ert starts, rather than when the forward model step runs, then that would reduce the amount of forward model failures we experience. Here is the list of commonly encountered errors:

* input file name does not exist
* invalid datetime in schedule
* invalid insert statement in config
* missing config file
* `File '../include/...' included via INCLUDE directive does not exist.`
* `IndexError` in /subscript/sunsch/time_vector.py", line 337, in _add_deck
* `Problem with keyword" in parsing the deck
* __pydantic_validator__ failing for config

And this, which may be a separate issue:

```
subscript/sunsch/sunsch.py", line 240, in
early_dates = [date for date in tmpschedule.dates if date.date() < startdate]
TypeError: can't compare datetime.datetime to datetime.date
```