Raise informative exception when metadata is unresolved in a metadata-based match

[MapleCCC]

I also tuned the related unit test to accommodate this change.

This PR closes #526.

[codecov-commenter]

Codecov Report

Merging #757 (5e5692f) into main (a077104) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #757   +/-   ##
=======================================
  Coverage   94.81%   94.81%           
=======================================
  Files         247      247           
  Lines       25802    25809    +7     
=======================================
+ Hits        24465    24472    +7     
  Misses       1337     1337           

Impacted Files	Coverage Δ
libcst/matchers/_matcher_base.py	91.30% <100.00%> (ø)
libcst/matchers/tests/test_findall.py	97.10% <100.00%> (ø)
...bcst/matchers/tests/test_matchers_with_metadata.py	100.00% <100.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[zsol]

The change looks great, thanks! Mind adding a quick note about this exception to the docs?

[MapleCCC]

Okay, in the new commit I documented this behavior to MatchMetadata and MatchMetadataIfTrue. 📚

[zsol]

Nice :)