Suggest "stack update" when encountering an unknown package

A friend of mine was stumped by this error message:

> hspec-parsec needed, but the stack configuration has no specified version (no package with that name found, perhaps there is a typo in a package's
> build-depends or an omission from the stack.yaml packages list?)

`hspec-parsec` is a new package, that wasn't yet included in her local package index. In this case a suggestion to run `stack update` would have been helpful.