feat!: improve mismatch error

This adds a new `Mismatch` class, with specific different kinds of
mismatches being subclasses of this class.

BREAKING CHANGE: The `srv.mismatches` is changed from a `list[dict[str,
Any]]` to a `list[Mismatch]`.

Signed-off-by: JP-Ellis <[email protected]>

Author: JP-Ellis