Behaviour of souffle with no output relations is not user friendly

[nkeynes-oracle]

Currently, running Souffle with a datalog specification that contains no output relations exits silently with no explanation or output. We should be at least generating a message along the lines of "No output relations found to generate".

Further confusing for users is the fact that turning debug on (with -d) unexpectedly changes the default behaviour and generates output for all relations; this is a change to behaviour that should probably be on its own flag.