Clean up error message on missing WARC

[tokee]

Moving WARC files or accidentally dismounting network drives is a common error. It does result in an Exception in solrwayback.log when resources are needed, but it is a very long stack trace which could be reduced to a simple FileNotFoundException so that people do not have to hunt for it.

Ideally this would bubble up to the GUI, but this calls for a more generic system. Related to #332 and #383