HooksMaster: Make RunPhase return the rpc output

Right now the hooks output is propagated from the nodes all the way up to
HooksMaster.RunPhase, which uses it for debugging PRE hooks, but then silently
discards them. We'll now propagate it up to the Processor.ExecOpCode function,
where they can be handled for other purposes (or discarded again, of course).
This patch also improves a bit the HooksMaster.RunPhase docstring.

Reviewed-by: iustinp