Add probes for extraction failure

[biancadanforth]

It would be valuable to know of all the supported product pages visited, what % is Fathom extraction successful and what % of fallback extraction is successful.

Some metrics to consider:

• Coverage: % detected products of all supported product pages
  • How to detect a product page?
• Extraction success/failure rate: % detected products by Fathom extraction
  • This will also give us the % detected products by fallback extraction

Note that this will not tell us whether the information extracted from the page is accurate.

[biancadanforth]

If we can successfully differentiate a product page from other kinds of pages on a supported site, then we may want to consider restricting extraction to only product pages as a part of closing this issue.

[biancadanforth]

Closing in favor of #158 .