Opportunity for a better error message when compiling an originally Elm project

[Janiczek]

User's intent: get Bytes-in-ports working in their Elm (not Lamdera) app

Steps to reproduce:

1. elm init
2. make src/Main.elm be a port module and contain a Bytes port (eg. port onConnectSuccessful : (Bytes -> msg) -> Sub msg), used in subscriptions.
3. lamdera make src/Main.elm

Expected: a nice error message about lamdera/core and lamdera/codecs missing in elm.json

Actual: a Map.! crash in here (something related to Wire decoders IIRC)