NEXUS-5472: REST changes and other

Changes:
* exposed isMavenRepositorySupported on WLManager, removed littered checks all over the place
* centralized and removed redundant checks for "state to be updated" (local state, proxy mode)
* all the update* methods are now throwing IllegalStateEx when a repo is tried to be WL updated but it's not in needed state(s)
* REST API now returns 400 Bad Request if WL Update on repo that is not in needed state tried.
NEXUS-5472: Cleanup.

More about illegal state and WL update, and javadoc. and one typo