Fix handling of argument to \set and others. (mathjax/MathJax#3447)

[dpvc]

This PR addresses a problem with the braket TeX package where arguments to macros like \set are not properly handled if the braces don't match. E.g., \set{ throws an untapped error, while \set} produces an empty set with no error.

These situations are handled by calling parser.GetArgument(name), which does the checking for balanced braces or missing arguments, but then putting the parser.i back to what it was. That way, the error messages will be produced when needed, and others the parsing is handled as it currently is.

The missing argument error message is no longer needed, as that is handled by the GetArgument() call.

Resolves issue mathjax/MathJax#3447.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.67%. Comparing base (59a9b65) to head (c98ed35).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1365      +/-   ##
===========================================
- Coverage    86.67%   86.67%   -0.01%     
===========================================
  Files          338      338              
  Lines        84268    84263       -5     
  Branches      4783     3150    -1633     
===========================================
- Hits         73039    73034       -5     
- Misses       11206    11229      +23     
+ Partials        23        0      -23     

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[zorkow]

lgtm.