Small fixes

[erkyrath]

Couple of improvements to diagnostics:

• If we encounter an internal error, we log the timestamp and URL before passing the error up to Koa.
• After loading Master-Index, log the number of cache entries found.

Also one serious fix. In cache.init(), if a single list_contents() call fails, the whole init() call fails. Then the cache tries to reload forever. Turns out list_contents() fails a lot! (See bug filed separately.)

I've updated cache.init() to catch these failures, log an error, and remove the offending file.

[erkyrath]

BTW, I've saved the logs as of today, so it's okay to wipe them by restarting the containers.

[curiousdannii]

Thanks, these all look appropriate.