Add testing for mismatch cases

This should help catch any errors in our handling of invalid cases.
While some of these produce outputs which aren't correct, what
we're checking here is that we don't _error_ while producing that
output.

Also fix a case which this showed up.

test/test_api/test_api.py

@@ -342,6 +342,7 @@ def test_file_fuzzy_completion(Script):
     script = Script('"{}/ep08_i'.format(path))
     expected = [
         'pep0484_basic.py"',
+        'pep0484_generic_mismatches.py"',
         'pep0484_generic_parameters.py"',
         'pep0484_generic_passthroughs.py"',
         'pep0484_typing.py"',