Install new error display handler to suppress stack traces for a86:user errors

[pdarragh]

Racket's raise-user-error function raises exceptions of the exn:fail:user variety, and the documentation states:

The default error display handler does not show a “stack trace” for exn:fail:user exceptions (see Continuation Marks), so raise-user-error should be used for errors that are intended for end users.

We have exn:a86:user as a category of errors. Either these should be changed to exn:user:a86, or we should adjust the error handler to display similarly for these as well.