Clarify Errors for Large Event Reports

When using api.add_event_report(), I get two errors.

1. When a MISPEvent is provided, api.get_uuid_or_id_from_abstract_misp() returns the UUID, but the add_event_report() for the event value must be the MISPEvent['id'] integer or string.
2. If the content value is larger than the database maximum (MySQL text max is 64k), the error provided is not useful.

Error 1:

Something went wrong (403): {'saved': False, 'name': 'Could not add EventReport', 'message': 'Could not add EventReport', 'url': '/event_reports/add', 'errors': 'numeric'}
{'errors': (403, {'saved': False, 'name': 'Could not add EventReport', 'message': 'Could not add EventReport', 'url': '/event_reports/add', 'errors': 'numeric'})}

Error 2:

Unknown error: the response is not in JSON.
Something is broken server-side, please send us everything that follows (careful with the auth key):
# content truncated with 100k content value
Response (if any):
{"name":"An Internal Error Has Occurred.","message":"An Internal Error Has Occurred.","url":"\/eventReports\/add\/1234"}
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "python3.10/site-packages/pymisp/api.py", line 486, in add_event_report
    new_event_report = self._check_json_response(r)
  File "python3.10/site-packages/pymisp/api.py", line 3497, in _check_json_response
    r = self._check_response(response, expect_json=True)
  File "python3.10/site-packages/pymisp/api.py", line 3514, in _check_response
    raise MISPServerError(f'Error code 500:\n{response.text}')
pymisp.exceptions.MISPServerError: Error code 500:
{"name":"An Internal Error Has Occurred.","message":"An Internal Error Has Occurred.","url":"\/eventReports\/add\/1234"}

Start in pymisp/api.py at add_event_report(), then follow _check_json_response() and _check_response() to understand the current error paths. Reproduce both cases from the issue: a MISPEvent passed as the event value and content exceeding the database limit. Done means the event identifier is accepted correctly and oversized content produces a useful, actionable error.