Add exception checks for file system.

source/microbit/fileobj.c

@@ -137,6 +137,7 @@ static mp_obj_t mp_builtin_open(size_t n_args, const mp_obj_t *args) {
     const char *filename = mp_obj_str_get_data(args[0], &name_len);
     RETURN_ON_EXCEPTION(MP_OBJ_NULL)
     file_descriptor_obj *res = microbit_file_open(filename, name_len, read == 0, text == 0);
+    RETURN_ON_EXCEPTION(MP_OBJ_NULL)
     if (res == NULL) {
         return mp_raise_msg_o(&mp_type_OSError, "file not found");
     }

source/microbit/filesystem.c

@@ -463,6 +463,7 @@ mp_lexer_t *microbit_file_lexer(qstr src_name, file_descriptor_obj *fd) {
 
 mp_lexer_t *mp_lexer_new_from_file(const char *filename) {
     file_descriptor_obj *fd = microbit_file_open(filename, strlen(filename), false, false);
+    RETURN_ON_EXCEPTION(NULL)
     if (fd == NULL)
         return NULL;
     qstr string = qstr_from_str(filename);