Check for required binaries before allowing start

[rvagg]

I got caught by this a few times, start should refuse to go if the binaries aren't there. It eventually fails but doesn't remind you to build, you have to know.

• build_status.rs has the list of binaries.
• prerequisites.rs only checks for lotus, it could check for all of them
• the prerequisites check runs late anyway, after portainer (LotusStep::pre_execute()), we should do that check earlier but look for all binaries (foc_devnet_bin().join(name).exists()) and ideally consolidate build_status.rs so it's shared

something like that.

[BigLep]

Similar to #29 (comment) I think. It would be great to check early that all prerequisites are satisfied.