Correct naming error in exception-showing
Correct naming error in exception-showing

Late edit to #437 missed a rename of `num` to `exc_num` - code was
showing external interrupts successfully, but trapped if faced with an
internal exception.

mbedmicro/pyOCD,mesheven/pyOCD,mbedmicro/pyOCD,mesheven/pyOCD,pyocd/pyOCD,mesheven/pyOCD,pyocd/pyOCD,flit/pyOCD,flit/pyOCD,mbedmicro/pyOCD