engine: unresolved placeholder in getDeviceVisibility error message

GetDeviceVisibility API verifies that all LUNs are connected to all
hosts in the data center. Up until now in case of failure this API
reported the error with unresolved placeholder '$hostName'. This patch
takes care to populate it's value.

Change-Id: I4b4b40a5296028fc8bcdc1f4a2ed156964a1b1a7
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>