Discovery: only retry join when other node is not (yet) a master

When a node tries to join a master, the master may not yet be ready to accept the join request. In such cases we retry sending the join request up to 3 times before going back to ping. To detect this the current logic uses ExceptionsHelper.unwrapCause(t) to unwrap the incoming RemoteTransportException and inspect it's source, looking for ElasticsearchIllegalStateException. However, local ElasticsearchIllegalStateException can also be thrown when the join process should be cancelled (i.e., node shut down). In this case we shouldn't retry.

This commit adds an explicit NotMasterException to indicate the remote node is not a master. A similarly named exception (but meaning something else) in the master fault detection code was given a better name. Also clean up some other exceptions while at it.

Closes #8972