Add share backup tests.

Share backup tests i.e. create/delete/get share_backups.

Change-Id: I366221702b3aa2e78bff82e6b2a543b6f6784f77
diff --git a/manila_tempest_tests/share_exceptions.py b/manila_tempest_tests/share_exceptions.py
index efa61b5..5b5ca18 100644
--- a/manila_tempest_tests/share_exceptions.py
+++ b/manila_tempest_tests/share_exceptions.py
@@ -86,3 +86,7 @@
 class ShareServerMigrationException(exceptions.TempestException):
     message = ("Share server %(server_id)s failed to migrate and is in ERROR "
                "status")
+
+
+class ShareBackupException(exceptions.TempestException):
+    message = ("Share backup %(backup_id)s failed and is in ERROR status")