webadmin: Added missing import cluster error message

Added the missing import cluster error message to frontend
AppErrors. Corresponding backend error message is already available.

Change-Id: I225adde40fdda99f7492a231ac210ca838655e6d
Bug-Url: https://bugzilla.redhat.com/884292
Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>