sync_ldap_user_data: Show a proper error if no relevant users exist.
sync_ldap_user_data: Show a proper error if no relevant users exist.

It doesn't make sense to run sync_ldap_user_data if user_profiles list
is empty. Otherwise this misleading exception gets raised:

```
raise Exception(
    "LDAP sync would have deactivated all users. This is most likely due "
    "to a misconfiguration of LDAP settings. Rolling back...\n"
    "Use the --force option if the mass deactivation is intended."
)
```

With some work by tabbott to manage the type of user_profiles and
provide a special error message for the empty server case.

rht/zulip,zulip/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,rht/zulip,andersk/zulip,zulip/zulip,andersk/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,andersk/zulip,rht/zulip,zulip/zulip,zulip/zulip