Description of problem:

When running `v7 plan', v7 would check required RPMs, if required RPMs are missing, it would try to install them, but it always gives the following error output even after installation succeeded:

Error: Could not install rpm 
"yum install -y xxxx" has output on stderr

This is because in V7_SRC/v7/command.py, 

 68         if self.errors and len(self.errors) > 0:
 69             raise V7CommandException(self, "has output on stderr")

And `yum install' will gives warning message via stderr:

This system is not registered with RHN.
RHN support will be disabled.
Warning: RPMDB altered outside of yum.

That causes error.

I doubt whether Line 68-69 are necessary for checking error, do we have any commands could be judged fail/success running only via stderr output?