nicer error reporting in repo.py.
refactor: base.sack is no longer a lazy attribute.

The client has to activate it manually with base.activate_sack(). This makes the
client code more explicit and removes apparent hacks allowed by on-first-access
semantics of the old attribute.
support repo.skip_if_unavailable config option. (RhBug:889202)

Updates the doc: with the newly enabled option.