fix(plugins): log stderr and stdout on non-zero return code

    Thu Sep 26 20:09:36 2013 DBG: (plugin=test) -> Metric added (dimension=nil, name=legacy_state, type=string, value=ok, unit=nil)
    Thu Sep 26 20:09:36 2013 DBG: (plugin=test) -> Setting check status string (status=)
    Thu Sep 26 20:09:36 2013 WRN: (plugin=test) -> Invalid metric line (line=metric blah) - Metric line not in the following format: metric <name> <type> <value> [<unit>]
    Thu Sep 26 20:09:36 2013 DBG: (plugin=test) -> /usr/lib/rackspace-monitoring-agent/plugins/test: done (code=1)
    Thu Sep 26 20:09:36 2013 DBG: (plugin=test) -> stdout: status ok
    Thu Sep 26 20:09:36 2013 DBG: (plugin=test) -> stdout: metric blah
    Thu Sep 26 20:09:36 2013 DBG: (plugin=test) -> stderr: message from stderr
    Thu Sep 26 20:09:36 2013 DBG: (plugin=test) -> check completed (id=chfroigruF, type=agent.plugin, state=unavailable)
    Thu Sep 26 20:09:36 2013 DBG: (plugin=test) -> reschedule check (id=chfroigruF, type=agent.plugin, state=unavailable)
    Thu Sep 26 20:09:36 2013 INF: (plugin=test) -> agent.plugin (id=chfroigruF, iid=idniu2vSqq, period=150s, args=(none)) scheduled for 150s