Lower log level or setting for missing files on origin

I'm happy to write up a patch but I'm not sure about the best approach. Specifically:

Should the level just be lower or should it be configurable?
Should this be only 40x responses or all non 200 responses?

"Problem" seems a bit overkill but I send error level logs to NewRelic and a flood of these trigger error thresholds. I also used this in a production site to "migrate" files from an old D6 where there was a _very_ large amount of cruft in the files directory but also random files linked from marketing emails, external marketing pages, etc that where hard to impossible to track down. That site also had random index bots or something that would hit it with garbage. The crying wolf started getting to me so I thought I'd file an issue. :)

I think a good argument could be made for them to be the same level as "access denied" and "page not found" which is warning.

I applied this and it didn't quite work as well as I expected. Or rather exposed a quirk of how the fetch works. Probably a bug.

FetchManager::fetch() logs messages when something fails and return false. Unless it doesn't know what happened and then it just return false. That mostly makes sense.

The code that calls fetch in ProxySubscriber then looks at the return. If its false it says "and unknown error occurred."

The problem is that basically all cases are handled and log specific errors already. That means if I'm reading everything correctly, there are 2 errors logged for most problem and the patch only lowers that to one.

I think the fix is just to move the message into the fetch method where its useful.