stm32/boardctrl: Skip running main.py if boot.py had an error.

Previous behaviour was: if boot.py had an exception then main.py would
still run, which is arguably unexpected behaviour.

This commit changes the behaviour so main.py is not run if boot.py has an
error.

Signed-off-by: Damien George <damien@micropython.org>

ports/stm32/boardctrl.c

@@ -155,6 +155,8 @@ int boardctrl_run_boot_py(boardctrl_state_t *state) {
             return BOARDCTRL_GOTO_SOFT_RESET_EXIT;
         }
         if (!ret) {
+            // There was an error, prevent main.py from running and flash LEDs.
+            state->reset_mode = BOARDCTRL_RESET_MODE_SAFE_MODE;
             flash_error(4);
         }
     }