Raise exceptions instead of using assert

[bouweandela]

There are assert statements in various places, e.g.
https://github.com/SciTools-incubator/iris-esmf-regrid/blob/fdd2051dfff050b18c356a2b4277426e74415a61/esmf_regrid/schemes.py#L185-L186

For applications and users of this library, it would be nice to replace those with exceptions with a message explaining the error.

[trexfeathers]

assert (S101) - Ruff (astral.sh)

[github-actions]

@SciTools/esmf-regrid-devs This issue is stale due to a lack of activity in the last 180 days. Remove stale label or comment, otherwise this issue will close automatically in 14 days time.

[stephenworsley]

Now captured by #437. (Ruff S101)