<check-sources.py> Set error/debug level to suppress all plugin messaging

Systems which lacked RHN registrations would repeatedly output the
"This system is not registered with..." message from the RHN
plugin. Setting the `YumBase` `preconf.errorlevel` and
`preconf.debuglevel` options to `-1` solves this by suppressing all
plugin-originating messaging.
Merge pull request #120 from dobbymoodge/check-sources-bugs

<check-sources.py> Set error/debug level to suppress all plugin messaging