Check if functions directory exists. Fixes #182

[zastavnitskiy]

- Summary

netlify dev --live fails to start if the functions directory doesn't exist.

Given the fact that netlify init doesn't create functions directory by default, the first-time experience of netlify dev is 😔

- Test plan

1. Init a project with netlify init
2. Try running netlify dev --live

- Description for the changelog

Don't assume functions directory, check if it exists.

[swyxio]

this is a fine solution. thanks!