Show merge errors in merge request widget
There were two problems here:
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
getMergeStatusmethod. Even if it wasn't, ensuring we have the right element means that the content gets updated.
On the Ruby side, the
log_merge_errormethod didn't update the
merge_errorcolumn 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#executeappears to be unused everywhere).