RBF: Exceptions are conflicting when creating the same mount entry twice

Test Steps:

1. Execute the command : hdfs dfsrouteradmin -add /apps7 hacluster /tmp7
2. Execute the same command once again.

Expected Result:

User should get the message saying already the mount entry is present.

Actual Result:

console message is displayed like below.

"Cannot add destination at hacluster /tmp7
Successfully added mount point /apps7"