libpq and error codes

I can't seem to find a reliable way to determine if the error from a
call to PQconnectdb is due to an invalid username/password. I can call
PQstatus and it tells me CONNECTION_BAD, which I'd expect, and then
PQerrorMessage gives me a plain text message that explains the reason, but
error messages can be localized. A different version of PostgreSQL may
decide to change the wording, so just doing a string comparison on the
error message won't cut it.

What I need is a nice, numeric error code that I can compare to a known
value that means invalid password. But there doesn't seem to be any
function to do that. Am I just being blind?

The reason I need this is that in our software, if the user tries to
open a connection to a database and gets an authorization error of some
kind, we pop up a username/password dialog and let them try again. But
if it's due to any other kind of problem (server down, etc), we give
them the error message directly.

Dave Williss