inform user if `buld` must be executed when doing codegen

[safareli]

At this moment if you running chanterelle codegen on project where chanterelle build is not been executed you would get:

$ chanterelle codegen
/Users/safareli/dev/foam/purescript-web3/test/chanterelle/output/Effect.Aff/foreign.js:532
                throw util.fromLeft(step);
                ^

Error: ENOENT: no such file or directory, open '/Users/safareli/dev/foam/purescript-web3/test/chanterelle/build/contracts/ComplexStorage.json'

It would be better to instead of this we say to user something like:

"hey you shuold run chanterelle build first"