vcs: Use more specific merge error reason (either target or source ref missing)

rhodecode/lib/vcs/backends/git/repository.py

@@ -867,7 +867,7 b' class GitRepository(BaseRepository):'
         except RepositoryError:
             log.exception('Failure when doing local fetch on git shadow repo')
             return MergeResponse(
-                False, False, None, MergeFailureReason.MISSING_COMMIT)
+                False, False, None, MergeFailureReason.MISSING_SOURCE_REF)
 
         merge_ref = None
         merge_failure_reason = MergeFailureReason.NONE

rhodecode/lib/vcs/backends/hg/repository.py

@@ -673,11 +673,16 b' class MercurialRepository(BaseRepository'
             return MergeResponse(
                 False, False, None, MergeFailureReason.TARGET_IS_NOT_HEAD)
 
+        try:
         if (target_ref.type == 'branch' and
                 len(self._heads(target_ref.name)) != 1):
             return MergeResponse(
                 False, False, None,
                 MergeFailureReason.HG_TARGET_HAS_MULTIPLE_HEADS)
+        except CommitDoesNotExistError as e:
+            log.exception('Failure when looking up branch heads on hg target')
+            return MergeResponse(
+                False, False, None, MergeFailureReason.MISSING_TARGET_REF)
 
         shadow_repo = self._get_shadow_instance(shadow_repository_path)
 
@@ -691,7 +696,7 b' class MercurialRepository(BaseRepository'
         except CommitDoesNotExistError:
             log.exception('Failure when doing local pull on hg shadow repo')
             return MergeResponse(
-                False, False, None, MergeFailureReason.MISSING_COMMIT)
+                False, False, None, MergeFailureReason.MISSING_SOURCE_REF)
 
         merge_ref = None
         merge_failure_reason = MergeFailureReason.NONE

rhodecode/tests/vcs/test_repository.py

@@ -401,10 +401,10 b' class TestRepositoryMerge:'
 
         assert merge_response == expected_merge_response
 
-    def test_merge_missing_commit(self, vcsbackend):
+    def test_merge_missing_source_reference(self, vcsbackend):
         self.prepare_for_success(vcsbackend)
         expected_merge_response = MergeResponse(
-            False, False, None, MergeFailureReason.MISSING_COMMIT)
+            False, False, None, MergeFailureReason.MISSING_SOURCE_REF)
 
         source_ref = Reference(
             self.source_ref.type, 'not_existing', self.source_ref.commit_id)