bin: undo upward state changes on children when a child fails

bin: undo upward state changes on children when a child fails

When a bin changes states upwards, and a child fails to change,
any child that was already switched will not be reset to its
original state, leaving its state inconsistent with the bin,
which does not change state due to the failure.

If the state change was from NULL to READY, it means that deleting
this bin will cause those children to be deleted while not in
NULL state, which is a Bad Thing. For other upward changes, it
is less of a problem, as a subsequent switch back to NULL will
cause an actual downwards change on those inconsistent elements,
albeit from the "wrong" state.

We now reset state to the original one when a child fails.

Includes unit test.

https://bugzilla.gnome.org/show_bug.cgi?id=747610