Log HTML rewriting errors

[Kixiron]

Logs any HTML rewriting errors to the console using log::error!() and increments a metric for failed html rewrites. Could be useful to see if we ever reach a point where we need to raise our memory limit

[jyn514]

A few points:

1. I don't expect to ever hit the memory limit in practice. From what @inikulin said in Switch from kuchiki to LOL_HTML #930 (comment), it seems like LOL uses basically no additional memory compared to storing the file in memory (❤️). However I'm ok with having this log just so we know that for a fact.
2. The log should be in the web module, not in the rewrite itself, i.e. it should match on the value instead of moving globals into the rewriter. Otherwise we're making it even harder to eventually add benchmarks.
3. This reasoning seems really strange to me:

It'd probably have roughly the same effect, but this makes sure that an error is always printed, regardless of if the error from the function is caught later on or not

Why should rewriting be special? If we want logging for requests that go wrong, we should add that directly instead of special-casing it here.

[Kixiron]

Addressed comments

[jyn514]

Looks good modulo nit :)