Fix crash when tigrc is absent

This fixes a regression introduced by 362a579 where the built-in tigrc
was not loaded because load_option_file() returned a custom error code
instead of ERROR_FILE_DOES_NOT_EXIST. The custom error message has been
moved to the tigrc 'source' command handler and load_option_file() has
been enhanced to only output a custom error when the errno != ENOENT.

Fixes #262