try except in InverseEnabler.draw_sample
just catching every error in the draw_sample function of the InversionEnabler class might shadow 'real' errors.
IMO only ValueErrors should be excepted.
(just had a problem and only got the 'cannot draw from inverse of this operator', which is weird, if the InversionEnabler is on. Also I was not drawing from the inverse.
The reason was, that the try clause caught a completely different error jumped into except where it tried to draw a sample from the inverse of said operator, which was not implemented for that operator)