Always raise exception if yum install fails.  This avoids later confusion when
packages are missing (the old code skipped 'Nothing to do' messages, but these
cases are still errors).

Fixes #5803
Signed-off-by: Warren Usui <warren.usui@inktank.com>
Reviewed by: Sandon Van Ness
Remove now unnecessary try/except/raise that came about as a result of
the previous check-in.

Fixes: #5803
Signed-off-by: Warren Usui <warren.usui@inktank.com>