Improve error handling in nonce code

Improve error handling in nonce code

Replace errno (from assuan code) by DBusError. Errors are not passed
through dbus_accept_with_nonce atm, as the original _dbus_accept() has
no error arg either.

Cherry-picked from commit 8082e92bb3a6304362e730f8b6e324860e1bb26c in
the dbus4win repository, edited to apply and fix whitespace issues by
tml@iki.fi.