Improve handling of webworker failures.

[dpvc]

This PR sets things up to better handle failures in landing the speech worker or its maps, rather than just crash.

In KeyExplorer.ts, we remove initial , (for when the speech is empty, as it will be if speech has failed). The Word/Math message is used when there should be speech, but speech hasn't been attached (i.e., speech has failed to be produced). This way, if the page is being read, you will at least get "math" as the output rather than nothing at all.

In speech.ts, the errors are always strings, not Error objects, so fixed that. In speechError(), we only report non-empty messages (this is to help avoid getting a speech error for every math expression on the page when SRE fails; see WebWorker.ts for more).

In GeneratorPool.ts, if the worker failed to start, we produce a message indicating that rather than no speech or braille. An alternative (in a comment) is to use the text content of the selected element, but that might be pretty ugly. On the other hand, it would at least be specific to the node being walked.

In WebWorker.ts, we only report a termination warning once if the warning is the same. That way, if the worker doesn't load, you don't get a redundant warning for every expression on the page, but just one warning. We also don't throw an error when terminating if the worker hasn't started (since we need to terminate the tasks). Finally, we add some commands to handle failure to start the worker, and failure to load a map file. We check to see if the error already includes the path that failed, and if not add it ourselves.

The speech-worker.ts file adds error handling to the getMap() methods so that errors are reported back to MathJax rather than just logged to the console.

Similarly, the HTMLAdaptor and NodeMixin adaptor are modified to include error trapping when the worker is started up.

Finally, the asyncLoad() function is fixed to handle node: references, which was lost somewhere along the line in the locale branch.

[codecov]

Codecov Report

❌ Patch coverage is 6.25000% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.92%. Comparing base (a8088bb) to head (68ed5f2).

Files with missing lines	Patch %	Lines
ts/adaptors/NodeMixin.ts	0.00%	8 Missing ⚠️
ts/adaptors/HTMLAdaptor.ts	0.00%	5 Missing ⚠️
ts/util/AsyncLoad.ts	33.33%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1538      +/-   ##
===========================================
- Coverage    86.93%   86.92%   -0.02%     
===========================================
  Files          388      388              
  Lines        87571    87584      +13     
  Branches      3290     3291       +1     
===========================================
+ Hits         76129    76130       +1     
- Misses       11442    11454      +12     

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