Release 6.20.0: Enhanced error handling in open-xchange-passwordchange-script

[changerequest]

The following error codes can now optionally be submitted by an underlying script:

- generic cannot change pasword <error code>
- password too short
- password too weak
- user not found
- ldap error

This is a compatible change, existing setups must not be changed.

This code has been contributed by Sameul Kvasnica.