[SSSD] [PATCH] subdomains: Fix error handling in Data Provider

[Jakub Hrozek]

To test, trigger any NSS operation with SSSD configured with a domain
that can't yield subdomains. The current git HEAD returns an error when
parsing the message returned from the Data Provider.

The patch also renames the "done" label to "immediate" because
"done"
is mostly used for both success and failure cases and refactors some
DEBUG statements that were too long to fit into the 80-characters limit
(enforced by our code guidelines as well as the fact that lines too long
don't fit into vertical split on my laptop screen)