Render valid message when MR source or target branch was removed

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/views/projects/merge_requests/_show.html.haml

@@ -2,7 +2,10 @@
   = render "projects/merge_requests/show/mr_title"
   = render "projects/merge_requests/show/how_to_merge"
   = render "projects/merge_requests/show/mr_box"
-  = render "projects/merge_requests/show/mr_accept"
+  - if @merge_request.source_branch_exists? && @merge_request.target_branch_exists?
+    = render "projects/merge_requests/show/mr_accept"
+  - else
+    = render "projects/merge_requests/show/no_accept"
   - if @merge_request.source_project.gitlab_ci?
     = render "projects/merge_requests/show/mr_ci"
   = render "projects/merge_requests/show/commits"

app/views/projects/merge_requests/show/_no_accept.html.haml

+.alert.alert-error
+  %p
+    This merge request can not be accepted because branch
+    - unless @merge_request.source_branch_exists?
+      %span.label.label-inverse= @merge_request.source_branch
+      does not exist in
+      %span.label.label-info= @merge_request.source_project_path
+    - else
+      %span.label.label-inverse= @merge_request.target_branch
+      does not exist in
+      %span.label.label-info= @merge_request.target_project_path
+  %strong Please close this merge request or change branches with existing one