Add error code for "invalid email/password"

[jeanbaptistebeau]

Error codes: https://github.com/realm/realm-core/blob/fd78e200dffe24a101ecdcacd691ace9607f63f5/src/realm/object-store/sync/generic_network_transport.hpp

The current error code for “invalid email/password” is -1 (unknown), which makes it indistinguishable from other errors. It would be nice to have a custom error code, to display the appropriate (localized) message to the user.