Print a traceback in case of error while running Disco-* features

PalaverService was sometimes swallowing errors while running Disco-*
features on the parent sub-services.

The reason why it was swallowing some errors but not all are ... dubious,
so I'm just removing the check so I can understand what's actually
happening.
Provide a better interface documentation for IStorage (still WIP)

I want to implement a custom IStorage than the ones provided by Palaver,
but the IStorage specification was, somehow, lacking.

I got some more information from actual implementation, completed the
interface with the methods implemented by backends (I mostly looked at the
pgsql_storage and dirdbm_storage) and try to guess the result returned
by each method.

There's still lot of work to do:

 * document parameters
 * document the interface when returning room information (it's currently a
   big dict with a very specific interface)
 * reword things so it's less clunky (I'm not sure how to word results
   returned by Deferred.)