Add checks to config

[Max-Meldrum]

As I noticed things may go very wrong if you are dumb and set threads/throughput to 0.

And for the num_cpus change:
rust-lang/futures-rs#1835

[Bathtor]

Please add some helpful error messages to these assertions.

Also, what happens if someone actually does want an empty thread pool and run everything dedicated for some reason. We should probably support this. Maybe with a special Scheduler implementation that throws exceptions on every scheduling attempt, so things don’t just quietly not work?

[Max-Meldrum]

Okay, will add error messages.

Well, if the threads is set to 0, the executors crate will panic as it has a check that threads > 0 when creating the pool.