Description of problem:
Nothing is logged if clear_netboot fails in beaker-proxy.

Version-Release number of selected component (if applicable):
0.14

Steps to Reproduce:
1. Break beaker-proxy's ability to run 'sudo beaker-clear-netboot', for example by commenting out the lines in /etc/sudoers.d/beaker_proxy_clear_netboot
2. Make a call to beaker-proxy to clear netboot for some system, using either XML-RPC or HTTP GET:

>>> import xmlrpclib
>>> s = xmlrpclib.ServerProxy('http://localhost:8000/RPC2')
>>> s.clear_netboot('testsystem.example.com')

>>> import requests
>>> requests.get('http://localhost:8000/nopxe/testsystem.example.com')

Actual results:
Clients receive an XML-RPC fault or HTTP 500 response, which is correct, but on the beaker-proxy side nothing useful is logged indicating that the request failed, or why.

Expected results:
Log messages in /var/log/beaker/proxy.log showing that the request failed, and why (in particular, the error message from sudo or beaker-clear-netboot if there was one).

Additional info:
Two problems here: failed XML-RPC calls are not logged at all, and failed HTTP requests are logged to stderr (which goes nowhere). Secondly, the actual error from sudo on its stderr is also not captured so it goes nowhere as well.