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).

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