Add better error propagation for rules during vectorization

[danielhertenstein]

We have found that during vectorization of pages (running the ruleset on a page using FathomFox), if a rule errors and never returns, the resulting vectors returned by Fathom are malformed. They may be empty (returning []) or Fathom may not return a set of vectors at all. This problem requires additional investigation to identify the complete behavior. Regardless, we need to add better error propagation so an error is returned to FathomFox instead of malformed vectors.

It is likely the work involved here will be helpful in completing #134 and #135.

See this comment chain for additional context: mozilla/fathom-fox#55 (comment)