engine: Improve error msg in vdsmErrors.properties

Improve error message of case when not being able
to access the master storage domain.
The fix is in VdsmErrors.properties

Change-Id: Iac0b46cfd1cd3f0bf353f5aecd5781b4baa14d6d
Bug-Url: https://bugzilla.redhat.com/975006
Signed-off-by: Alissa Bonas <abonas@redhat.com>