py/runtime: Fix crash when exc __new__ doesn't return an exc instance.
py/runtime: Fix crash when exc __new__ doesn't return an exc instance.

See CPython bug https://bugs.python.org/issue39091 for more details.

adafruit/circuitpython,adafruit/circuitpython,bvernoux/micropython,bvernoux/micropython,bvernoux/micropython,adafruit/circuitpython,adafruit/circuitpython,bvernoux/micropython,adafruit/circuitpython,adafruit/circuitpython,bvernoux/micropython