Show merge errors in merge request widget

There were two problems here:

1. On the JS side, the reference to $widgetBody didn't refer to the
   right DOM element any more. This might be because it was replaced by
   the `getMergeStatus` method. Even if it wasn't, ensuring we have the
   right element means that the content gets updated.

2. On the Ruby side, the `log_merge_error` method didn't update the
   `merge_error` column of the merge request. Change that to update if
   requested, and update in the most common cases by default.

   Additionally, this would sometimes return an error hash, but it
   doesn't look like this was ever used (the return value of
   `MergeService#execute` appears to be unused everywhere).

1 files changed, 10 insertions, 8 deletions

diff --git a/app/services/merge_requests/merge_service.rb b/app/services/merge_requests/merge_service.rb
index 5ca6fec962d..177b714b734 100644
--- a/app/services/merge_requests/merge_service.rb
+++ b/app/services/merge_requests/merge_service.rb
@@ -11,18 +11,20 @@ module MergeRequests
     def execute(merge_request)
       @merge_request = merge_request
 
-      return log_merge_error('Merge request is not mergeable', true) unless @merge_request.mergeable?
+      unless @merge_request.mergeable?
+        return log_merge_error('Merge request is not mergeable', save_message_on_model: true)
+      end
 
       @source = find_merge_source
 
-      return log_merge_error('No source for merge', true) unless @source
+      unless @source
+        log_merge_error('No source for merge', save_message_on_model: true)
+      end
 
       merge_request.in_locked_state do
         if commit
           after_merge
           success
-        else
-          log_merge_error('Can not merge changes', true)
         end
       end
     end
@@ -43,11 +45,11 @@ module MergeRequests
       if commit_id
         merge_request.update(merge_commit_sha: commit_id)
       else
-        merge_request.update(merge_error: 'Conflicts detected during merge')
+        log_merge_error('Conflicts detected during merge', save_message_on_model: true)
         false
       end
     rescue GitHooksService::PreReceiveError => e
-      merge_request.update(merge_error: e.message)
+      log_merge_error(e.message, save_message_on_model: true)
       false
     rescue StandardError => e
       merge_request.update(merge_error: "Something went wrong during merge: #{e.message}")
@@ -70,10 +72,10 @@ module MergeRequests
       @merge_request.force_remove_source_branch? ? @merge_request.author : current_user
     end
 
-    def log_merge_error(message, http_error = false)
+    def log_merge_error(message, save_message_on_model: false)
       Rails.logger.error("MergeService ERROR: #{merge_request_info} - #{message}")
 
-      error(message) if http_error
+      @merge_request.update(merge_error: message) if save_message_on_model
     end
 
     def merge_request_info