audit suggestion: track runtime errors and warnings

[alekseykulikov]

Tracking this, in the way Lighthouse tracks user timing metrics may help in different cases, for example:

• Having runtime exceptions can be a bad sign, that some javascript loaded on the pages fails, and time to load/parse is wasted.
• warnings (console.warn) may indicate that page uses deprecated/outdated technology, that may have vulnerabilities or hurt performance.

If this is a valuable suggestion, I can try to provide a PR.
This issue may also be related to #1512

[patrickhulce]

someone else thought so too :) #2836

deprecations should already be surfaced by the "deprecations" audit that uses the console messages as the source.

[patrickhulce]

fixed by #2836