libdwfl: Use actual file names in parse_opt failure cases.

libdwfl: Use actual file names in parse_opt failure cases.

When handling ARGP_KEY_SUCCESS the failure handling code would use 'arg'
as failure string. But 'arg' would be NULL in such cases leading to failure
messages like: "cannot open '(null)': No such file or directory".
Use opt->e and opt->core explicitly to show file names that caused failure.

Signed-off-by: Mark Wielaard <mjw@redhat.com>