It's happening because do_source is only expected to return FAIL when it
was unable to open file . But `nlua_exec_file` returns fail for parsing
and execution error too . Those errors are emitted through `nlua_error`.
So now return value of nlua_exec_file is ignored like do_cmdline. It now
only returns fail when it was unable to open file that check is done
before calling nlua_exec_file or do_cmdline. Errors in nlua_exec_file
are still directly emitted through nlua_error like before.
