[now dev] Centralize error responses logic (#2026)
[now dev] Centralize error responses logic (#2026)

This just DRYs up the code a bit and centralizes the error sending logic
so that we can more easily add the HTML and JSON versions of the
responses as well.
zeit/now-cli,zeit/now-cli,zeit/now-cli,zeit/now-cli,zeit/now-cli,zeit/now-cli,zeit/now-cli