Add new --equivalent ghostwriter which allows same-typed exceptions

[Zac-HD]

The Ghostwriter often writes over-broad input strategies, which we accept on the basis that the user should be responsible for narrowing this down.

However, for the --equivalent property, we might (optionally) be OK with tested functions raising an exception - so long as they raise the same exception type. I'd implement this with an additional argument in the Python API, and a special value which can be passed via --except=<some special value> on the CLI. (still need to think of a good name for that)