Refactoring: Use soft error reporting for *_opt_error functions

Hi,

While reviewing version v6 of the CAST(... ON DEFAULT) patch [1], I
noticed that it attempts to change the type conversion function to use
soft error reporting. However, some of the underlying functions of the
type conversion, such as *_opt_error, still rely on a boolean argument
passed by the caller. This results in an error flag being set rather
than a proper error being thrown.

I believe we should update all *_opt_error functions to use the new
soft error reporting infrastructure instead of boolean flags -- did
the same in the attached patch. I am not sure if this patch should be
part of that thread[1]. It's a significant improvement in itself, as
it would make the code more compact and consistent.

1]http://postgr.es/m/CACJufxE053=bO3pDUpGba6Yz3VGpU_XCbg4HO6Rew5EJ7k7VnQ@mail.gmail.com

--
Regards,
Amul Sul
EDB: http://www.enterprisedb.com