Raise internal error for nonexistent subpackages

[MridulS]

A hacky fix for #9, not sure if this the best way. Maybe it's okay to just keep it as it is? pytest seems to consider zero collected tests also as a special case, https://docs.pytest.org/en/stable/reference/exit-codes.html

Fixes #9

[pllim]

The failing CI isn't a good sign.

I actually think "0 test collected" (current behavior without this PR) is reasonable, so personally I don't think #9 is critical to address, but maybe @bsipocz feels otherwise still.

[MridulS]

Ah yeah, this only seems to works with pytest > 8.

I agree maybe it's just fine to keep the current behavior? (Especially because I don't really like the solution I proposed 😅 )

[pllim]

Thanks for trying!