vcs: Add merge failure reasons for missing target ref and missing source ref.

rhodecode/lib/vcs/backends/base.py

@@ -97,6 +97,12 b' class MergeFailureReason(object):'
     # A involved commit could not be found.
     MISSING_COMMIT = 8
 
+    # The target repo reference is missing.
+    MISSING_TARGET_REF = 9
+
+    # The source repo reference is missing.
+    MISSING_SOURCE_REF = 10
+
 
 class BaseRepository(object):
     """