[REVIEW] NEXUS-5511: Improve not found reasoning

Nexus might response with 404 Not Found in for many reasons, and proxy404 will be "just another" one of them.

Nexus since 2.2 (or 2.3?) bubbles up the actual not found message to REST layer and HTML content page even renders the message to user. But, unfortunately, the message was a constant ("The item not found in ...").

These changes now changes critical places like NFC, local storage, remote storage, processors to give more hints why response is 404.

Aside improving messages, this pull also gives ability to "hint" reason upfront, like the case with localOnly or groupLocalOnly flags is (by setting these flags you still don't know the outcome of the response). A similar technique would be used in proxy404 to tell user that "Whitelist rejected to perform a remote request".