[$] Corner cases and exception types

Some unanticipated corner cases with Python's new "walrus"operator-described in our Python 3.8overview-have cropped up recently. The problematic uses of the operatorwill be turned into errors before the final release, but just whatexception should be raised came into question. It seems that the exceptionspecified in the PEP for the operator may not really be the best choice, as arecent discussion hashed out.