Make the retry error message more descriptive

[dpvc]

This PR adds more detail to the "MathJax retry" error message so that programmers who use the synchronous functions and don't trap this error will get more details about what is happening. (There is also a completely new section in the v4 documentation about this error, so I'm hoping this PR and that new page will reduce the questions about this.)

The error message is never actually used internally (just the .retry property), so the actual message is immaterial from the standpoint of the code.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.71%. Comparing base (4d1e195) to head (8f732da).
Report is 32 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1303   +/-   ##
========================================
  Coverage    86.71%   86.71%           
========================================
  Files          337      337           
  Lines        83979    83982    +3     
  Branches      4750     4750           
========================================
+ Hits         72826    72829    +3     
- Misses       11130    11153   +23     
+ Partials        23        0   -23     

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[zorkow]

lgtm.