Improve error handling in REGISTER command

[rtobar]

As noted by @smclay, and years ago by myself, the REGISTER command doesn't always return a failure to the user when file registration files. In particular, some errors in the _registerExec function are simply logged and then reported via email, but not reported back to the client originating the request. _registerExec can be executed either during the original HTTP request, or in the background, and therefore it was probably originally thought that raising exceptions not always resulted in user-visible errors.

This situation should be improved, with users receiving errors when they happen.