Fix name of non-fatal cache exception. Resolves:
```WARNING:pants.cache.restful_artifact_cache:
Error while reading from remote artifact cache: global name 'NonfatalCacheError' is not defined```

Testing Done:
./pants goal test tests/python/pants_test/cache::, but there's no actual coverage on these errors (yet).

Bugs closed: 764

Reviewed at https://rbcommons.com/s/twitter/r/1274/