* Add crawl errors endpoint
If this endpoint is called while the crawl is running, errors are
pulled directly from redis.
If this endpoint is called when the crawl is finished, errors are
pulled from mongodb, where they're written when crawls complete.
* Add nightly backend test for errors endpoint
* Add errors for failed and cancelled crawls to mongo
Co-authored-by: Ilya Kreymer <ikreymer@users.noreply.github.com>
