Detect errors before reporting files that are no longer referenced

[tidoust]

The action that runs to report files that are no longer referenced raises false positives when crawling errors occurred. Sometimes, these errors are not really detected as errors and there is not much we can do about that apart from manually checking the result. However, in some cases, the errors are actually detected and reported in the index.json file. The action should check these and either not run when errors were detected or skip over files that seem linked to the specs that errored.

(Inspired by #375 (review))