[REVIEW] NEXUS-5511 Refine reasoning

Original fix for NEXUS-5511, but left some loose endings.

Conditions like "request is local-only", "proxy mode is not ALLOW" and "rejected by auto-routing" were all put under same message stating "...remote storage access is prevented...". Clearly, this is no good.

This pull separates now these scenarios, but also improves the messages by aligning how repository is "rendered" (unifying the message).

See issue for "validation scenario" where new (but also old) reasoning messages are represented:
https://issues.sonatype.org/browse/NEXUS-5511