subrepo: Add merge failure reason code ad message for subrepo merge conflicts.

rhodecode/lib/vcs/backends/base.py

@@ -104,6 +104,10 b' class MergeFailureReason(object):'
     # The source repo reference is missing.
     MISSING_SOURCE_REF = 10
 
+    # The merge was not successful, there are conflicts related to sub
+    # repositories.
+    SUBREPO_MERGE_FAILED = 11
+
 
 class UpdateFailureReason(object):
     """

rhodecode/model/pull_request.py

@@ -105,6 +105,9 b' class PullRequestModel(BaseModel):'
         MergeFailureReason.MISSING_SOURCE_REF: lazy_ugettext(
             'This pull request cannot be merged because the source '
             'reference is missing.'),
+        MergeFailureReason.SUBREPO_MERGE_FAILED: lazy_ugettext(
+            'This pull request cannot be merged because of conflicts related '
+            'to sub repositories.'),
     }
 
     UPDATE_STATUS_MESSAGES = {