Warn if no partitions are specified during `rojo serve`

[LPGhatguy]

When people first pick up Rojo, they often forget to add partitions.

rojo serve should issue a warning if there aren't any specified!

[LPGhatguy]

Coming soon to a build near you:

$ rojo serve
Using project "server" from C:\Users\lgreathouse\projects\rojo\server

This project has no partitions and will not do anything when served!
This is usually a mistake -- edit rojo.json!

Server listening on port 8000