8042/hotfix/false success on author merge

[benbdeitch]

Closes #8042

Hotfix, so that unsuccessful merges don't display as successful.

Technical

The Ajax request for merges was set to always load in the 'successful merge element' on completion, regardless of whether or not the merge was successful. It has been split into a 'success' function and an 'error' function respectively.

Testing

Simply attempt an impossible merge, and an error message should show up.

Screenshot

Stakeholders

@cdrini

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (d376947) 16.62% compared to head (cd6ef8f) 16.61%.
Report is 3 commits behind head on master.

Files	Patch %	Lines
openlibrary/plugins/openlibrary/js/merge.js	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8735      +/-   ##
==========================================
- Coverage   16.62%   16.61%   -0.02%     
==========================================
  Files          88       88              
  Lines        4698     4701       +3     
  Branches      837      837              
==========================================
  Hits          781      781              
- Misses       3399     3402       +3     
  Partials      518      518              

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[jimchamp]

Thanks, @benbdeitch, and sorry for the delay.
This seems to be working. I'm going to remove that confusing comment and merge this.