Improve out-of-minors handling

Currently, the out-of-minors handling is not very good: though both MD
and DRBD functions for finding an unused minor can return None, only the
DRBD code checks for this case.

This patch improves this by making the functions _FindUnusedMinor raise
an error instead of None, and gets rid of the one manual check for the
return value.

Reviewed-by: imsnah