fix mruby command returning EXIT_SUCCESS when program file can't be
opened.
remove unused variable

GLOBAL_RAND_SEED_KEY is no longer used.
Merge pull request #1807 from tmash06/fix_exit_status_code

fix mruby command returning EXIT_SUCCESS when program file can't be opened.
Merge pull request #1808 from cubicdaiya/issues/unused_variable

remove unused variable