core: Throwing exception instead of handle fault

CommandBase -> executeWithoutTransaction already handles a
VdcBLLException by calling processExceptionToClient.
So AddStorageServerConnectionCommand should throw an
exception with a right error message, and this way both UI and REST api
get the same response.

Change-Id: I3e6b8282d9e2cdd6e17f21a2ef034041d68d09fc
Bug-Url: https://bugzilla.redhat.com/1108164
Signed-off-by: Idan Shaby <ishaby@redhat.com>