Wrap _main with a sys.exit and come up with a proper exception handling strategy

[Patater]

From @rwalton-arm in https://github.com/ARMmbed/mbed-os-tf-m-regression-tests/pull/101#discussion_r622132929les "If I ever want to call this from some other code it could possibly exit the interpreter without giving me any chance to handle the error, which isn't very considerate."