Bug#472849: More descriptive error

[Christophe TROESTLER]

For having been bitten by this, I second the wish of a more
descriptive error message.  The splitting of modules seems to be a
nice solution to me.  Most users will want the plugins however, so
what about having rhythmbox (virtual package) depend on
rhythmbox-player and rhythmbox-plugins, the latter depending on
python-gst0.10?

Thanks for your work,
ChriS