Error flags set on Drivers and F-Curves when they can't be evaluated or flushed
properly are now cleared when loading files, allowing drivers to be re-evaluated
when a file is reloaded. This means that if a driver couldn't be used in the
previous session due to the set of extension modules active at the time (and was
thus disabled), reloading the file with the necessary extensions loaded means
that the driver can work out of the box without manually re-enabling.
e92fea3efb