Error message of changePassword is wrong when external user (e.g. LDAP user) tries to change password

[florianlink]

The error is always "Old password is incorrect."
If the user has not password in the model, the error should be:
"External user can not change password."

[florianlink]

It would also make sense to:

The user documents that are returned when using GET /user/ should contain a flag "isExternal" to indicate that the user is an external user and has no password/salt in the Girder DB. This would be useful for not showing these users a ChangePassword dialog in our application. The flag would have to be set to true when the user is created e.g. by the LDAP plugin.