Validation errors were behaving too much like real errors: they were
being printed with stack traces, and passed to the options(shiny.error)
function. Also, if a reactive() cached a validation error, on future
calls the error would be re-raised (which is correct) without the
custom class names attached (which is not).
No known errors will occur when printing the stack trace, but since
this is fairly complicated code executing when an error has already
happened, better to be on the defensive.
- More selective removal of frames at end of call stack
- Add withLogErrors, printError, stripStackTrace convenience funcs
- Properly capture/log errors for various levels of unhandled errors
- Unhide stacks for flush/flushed/sessionended callbacks
