JBIDE-15505 - Exception in JAXRS at startup

Added a check for null metamodel before calling the JaxrsMetamodel#removeListener() method
JBIDE-15599 - Changes on a subresource locator fields are not propagated to the JAX-RS Explorer

Registered the URI Mappings Content Provider (in the UI) when the JAX-RS Web Services node is
expanded. This avoids the case where in some circumstances, the JAX-RS metamodel would have no
listener to notify when JAX-RS endpoints changed.

Also edited the log messages for sake of readability