py: Check for valid file when creating lexer for execfile.

Addresses issue #1119.

py/builtinevex.c

@@ -134,6 +134,9 @@ STATIC mp_obj_t eval_exec_helper(mp_uint_t n_args, const mp_obj_t *args, mp_pars
     mp_lexer_t *lex;
     if (MICROPY_PY_BUILTINS_EXECFILE && parse_input_kind == MP_PARSE_SINGLE_INPUT) {
         lex = mp_lexer_new_from_file(str);
+        if (lex == NULL) {
+            nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_OSError, "could not open file '%s'", str));
+        }
         parse_input_kind = MP_PARSE_FILE_INPUT;
     } else {
         lex = mp_lexer_new_from_str_len(MP_QSTR__lt_string_gt_, str, str_len, 0);