webadmin: Fix the phrasing of error message

message was: Cannot edit VM. Dedicated Host Cluster is not identical to
VM Cluster.

changed to: Cannot edit VM. VM is pinned to a specific host. The host's
cluster must be the same as the selected VM cluster.

Change-Id: I298bacc9ce5c2ca9729d79ee3136a71360d329d5
Bug-Url: https://bugzilla.redhat.com/950693
Signed-off-by: Ofri Masad <omasad@redhat.com>