argpar.c: add error enumerators to `enum parse_orig_arg_opt_ret`

argpar.c: add error enumerators to `enum parse_orig_arg_opt_ret`

This patch changes `enum parse_orig_arg_opt_ret` to add

* `PARSE_ORIG_ARG_OPT_RET_ERROR_MISSING_OPT_ARG`
* `PARSE_ORIG_ARG_OPT_RET_ERROR_INVALID_OPT`
* `PARSE_ORIG_ARG_OPT_RET_ERROR_UNEXPECTED_OPT_ARG`
* `PARSE_ORIG_ARG_OPT_RET_ERROR_MEMORY`

and remove the general `PARSE_ORIG_ARG_OPT_RET_ERROR`.

No functional changes intended.

This will make it easier to switch to rich error reporting instead of
a formatted error string.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I2e68bea8b14ea1c0b16433df0cb8c01c1252283d