py: Add mp_raise_msg_varg helper and use it where appropriate.
This commit adds mp_raise_msg_varg(type, fmt, ...) as a helper for
nlr_raise(mp_obj_new_exception_msg_varg(type, fmt, ...)). It makes the
C-level API for raising exceptions more consistent, and reduces code size
on most ports:
bare-arm: +28 +0.042%
minimal x86: +100 +0.067%
unix x64: -56 -0.011%
unix nanbox: -300 -0.068%
stm32: -204 -0.054% PYBV10
cc3200: +0 +0.000%
esp8266: -64 -0.010% GENERIC
esp32: -104 -0.007% GENERIC
nrf: -136 -0.094% pca10040
samd: +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS
