XHR: send HTTP 400 error for invalid entry point

Returning an HTTP error code from xmlhttprequest back to the caller
instead of just outputting some arbitrary text message ensures that the
client's call actually fails, without relying on the failure being
caused by mismatch in output type, or detecting the error by parsing the
output.

HTTP 400 (Bad Request) error code was chosen following discussion in
pull request https://github.com/mantisbt/mantisbt/pull/1139.

Fixes 0023184