Better error message for BCE years

[movermeyer]

What are you trying to accomplish?

Fixes #156

What approach did you choose and why?

Improved the error message specifically for the cases where someone attempts to parse BCE years.

What should reviewers focus on?

🤷 Our performance guarantees are on the happy path. On the error path, we try to give the best error messages possible.

The impact of these changes

Users will be be given a better error message in this specific case.

Testing

>>> from ciso8601 import parse_datetime
>>> parse_datetime("-2014-01-02")
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    parse_datetime("-2014-01-02")
    ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
ValueError: Invalid character while parsing year ('-', Index: 0). While valid ISO 8601 years, BCE years are not supported by Python's `datetime` objects.