Better handling of storage errors

[na--]

Currently, when there are errors with the cache storage, nedomi fails almost silently. Only some log entries are written. The client receives empty responses with status code 200 and the nedomi status page does not show the errors in any way. We should have a better way to monitor such problems and we should proxy the request to the upstream if there are problems with the cache storage.

[ironsmile]

Is this relevant anymore?

[na--]

2/3 done, I think. A lot of storrage errors are logged in the application log, but we still show no storage health information in the status page. I think even the percent of cache hits still increments, even if there is a storage error.