Bugfix [#32492] - Part 3: Error filter now includes drivers where there are
errors with the variables/targets, even if those errors are for variables which
aren't used (and are hence "harmless" errors)

This means that the filter can be truly useful for helping locate things that
need "cleaning up". For example, previously, there could still have been drivers
where there were some of these "harmless" errors would emit warnings, but would
otherwise appear perfectly functional.

The implementation here uses a slightly slower method of checking any errors in
these driver vars. However, it's no slower than what's done when these are
evaluated, and should be less error prone than introducing yet another type of
error tagging for this one case. The problem here is that the "driver invalid"
flag, which is usually set when a target has errors, gets cleared by the
pydrivers code if nothing went wrong when evaluating the expression. Removing
this clearing step will probably open a can of worms, so unless this method
proves to be far too slow, this simpler fix will do.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55070 954f8c5b-7b00-dc11-b283-0030488c597c