Be a smidge more error tolerant?

[thejcannon]

While playing around with using setuptools_scm I hit more than a few AssertionErrors with the only context being the code and traceback.

Perhaps these could be improved with an assertion message, or even a ValueError instead 😄

[RonnyPfannschmidt]

Absolutely

[thejcannon]

FYI I'll re-create those situations and bake you a perfectly crisp PR 🧑‍🍳