Show pre-receive error message in merge request widget

When a merge fails due to a locked file, no feedback was given to the
user as to why it failed if the user used the Merge button.  However, if
the push attempt happened directly in the Web UI, the pre-receive
message would be displayed.

To be consistent and to make it clear why a merge failed, we now include
the pre-receive error message in the merge request widget.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/10165

app/services/merge_requests/merge_service.rb

@@ -76,8 +76,7 @@ module MergeRequests
     def try_merge
       repository.merge(current_user, source, merge_request, commit_message)
     rescue Gitlab::Git::PreReceiveError => e
-      handle_merge_error(log_message: e.message)
-      raise_error('Something went wrong during merge pre-receive hook')
+      raise_error("Something went wrong during merge pre-receive hook: #{e.message}")
     rescue => e
       handle_merge_error(log_message: e.message)
       raise_error('Something went wrong during merge')

changelogs/unreleased/sh-show-pre-receive-error-merge.yml

+---
+title: Show pre-receive error message when merge fails
+merge_request: 25708
+author:
+type: fixed

spec/services/merge_requests/merge_service_spec.rb

@@ -244,7 +244,7 @@ describe MergeRequests::MergeService do
 
         service.execute(merge_request)
 
-        expect(merge_request.merge_error).to include('Something went wrong during merge pre-receive hook')
+        expect(merge_request.merge_error).to include("Something went wrong during merge pre-receive hook: #{error_message}")
         expect(Rails.logger).to have_received(:error).with(a_string_matching(error_message))
       end