Add button to retry the merge request when some error occurs during the merge

Description

If something goes wrong during the merge of a merge request, an error with the reason is shown on the merge request widget. Currently, the unique way to retry the merge is to do a force push. We should have a button on the merge request widget to allow the user to try it again, see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10965#note_28430644.

/cc @smcgivern