FillPDF Service errors are exposed to user

One somewhat benign, somewhat jarring behavior in Fill PDF at the moment is that the end user sees errors they have no way of resolving. These should be replaced with a more generic message such as, "An unexpected error has occurred, and the administrator has been notified. If the error persists, provide them with error code (something or other)."

And then log the detailed message to watchdog. And document the error codes somewhere so they're searchable.