Improve get_arg_covariant_or_fail exception text

[nfriendly]

The exception throw no get_arg_covariant_or_fail does not include the expected prefix. This makes debugging the error more difficult because you don't know exactly what the fixture was searching for.

https://nanaimo.readthedocs.io/en/latest/_modules/nanaimo/fixtures.html#Fixture.get_arg_covariant_or_fail