Detect even more Yum errors

Today we had a funny transaction that reported missing files and looked
like this:

************************************************************
ERROR
<list of files>
Transaction encountered a serious error
************************************************************

And it didn't match our regular expression.  So here comes yet another
attempt to detect every possible error that Yum hides from us.
Merge pull request #78 from piojo/more-error-handling

Detect even more Yum errors