cleanup errors messages during installation

cleanup errors messages during installation due to various config
are not created. they are handled as warnings and will not return any
config information.
skip updating if grub2-mkconfig not found

In update-bootloader, we should skip updating grub2 if there's no
grub2-mkconfig utility found. This is a protection from running
update-bootloader while no corresponding grub2 package gets installed
(bnc#858268).
Merge pull request #21 from WenhuaChang/master

cleanup errors messages during installation