Merge request error display.

Fixes #8432

app/services/merge_requests/build_service.rb

@@ -13,7 +13,7 @@ module MergeRequests
       merge_request.target_branch ||= merge_request.target_project.default_branch
 
       unless merge_request.target_branch && merge_request.source_branch
-        return build_failed(merge_request, "You must select source and target branches")
+        return build_failed(merge_request, nil)
       end
 
       # Generate suggested MR title based on source branch name
@@ -59,7 +59,7 @@ module MergeRequests
     end
 
     def build_failed(merge_request, message)
-      merge_request.errors.add(:base, message)
+      merge_request.errors.add(:base, message) unless message.nil?
       merge_request.compare_commits = []
       merge_request.can_be_created = false
       merge_request