inbound request URL syntax validity should be checked and fail fast

Inbound request URLs that contain certain invalid characters may bypass basic syntax checking and cause Nexus to perform more internal work then necessary. This can also cause Nexus to report noisy WARN messages in the logs.

Expected: Nexus should perform URL syntax validation as early as possible and fail with simple log messages instead of verbose WARN when a URL is syntactically invalid. The HTTP response should return a 400 status in this case.