fence-rhevm: improve error reporting.

fence_rhevm uses REST API to call RHV

In fence_rhevm , the plug parameter is actually the VN name and is used
in order to construct a REST API search request to get the VM UUID from
RHV by the VM name.
In case that the call fails an unrelated error is displayed

"Failed: Unable to obtain correct plug status or plug is not available"

Code should be changed to output a clear error in case that the REST API fail

Issue : #225
Signed-off-by: emesika emesika@redhat.com