Currently we log tracebacks on cleaning if a System doesn't have
a Bios attribute (seems the case for some Seamicro systems). This
patch turns it into a warning and provides correct handling of
such cases in other BIOS calls.
Also cleans up mocking in the BIOS tests since the current approach
can yield surprising results with error inheritance.
Change-Id: I34569806c3d34628b688661a3796e8d9b394135c