0.6dev: Switching `master.py` to use new `self._send_response()` and `self._send_error() methods. Simplifies code, but most importantly for errors it allows a consistent method for transmitting plain-text error messages to the slave (that the slave will now output as part of debug logging). Raising Trac HTTP* errors actually causes full rendering of an HTML error page as response, which is both inefficient as well as making it near-impossible to extract the 'hidden' message from the master.