Add error enumerators to `enum argpar_iter_parse_next_status`

Add error enumerators to `enum argpar_iter_parse_next_status`

This patch adds all the possible reasons for a failure of
argpar_iter_parse_next() as error enumerators to
`enum argpar_iter_parse_next_status`.

This will make it possible to convert the `error` parameter of
argpar_iter_parse_next() to a rich error, without any formatted error
message.

In `test_argpar.c`, test_fail() now accepts an expected
argpar_iter_parse_next() status which it forwards to
test_fail_argpar_iter() which ensures that argpar_iter_parse_next()
returns the expected error status.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I9e8e91dea6066bd2ee935d6ca465569a6fd379aa