* Expose core error logger to plugins and report custom criteria errors
Fixes#77
This adds an error logging method on the IObservatoryCore interface that writes the exception details to ObservatoryCore's central error log (found in `${Documents}/ObservatoryErrorLog.txt`). In addition, added a timestamp to each error log.
Also updates the Explorer to report Custom Criteria file load errors and execution errors to the log. Also updates HeraldNotifier to report CacheIndex.json parse failures to the error log as well.
* Expand debugging/error logging in Herald; cleanup empty mp3 files
Herald crashes if attempting to play 0-byte mp3s so if detected, delete, re-request (empty files can occur in some azure failure cases (ie. out of quota). Trap and log errors in other places in HeraldQueue to avoid hard crashes due to weird and wonderful unexpected stuff.
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