Fix Elm transform showing compiler errors twice

[JensPfeifle]

↪️ Pull Request

Fix for #6703 by wrapping the Exception thrown by compileToString in a ThrowableDiagnostic.
Unfortunately this doesn't seem to fix #6124.

I looked into using codeframes to get syntax highlighting. Running the elm compiler with --report=json gives usable diagnostic location information, but the error message is still quite verbose, including code snippets (again) and hints. It just didn't look right, so I've left it at printing the stack trace.

💻 Examples

Before

After

🚨 Test instructions

Start parcel v2 with any broken Elm code, e.g.
Main.elm:

module Main exposing (main)
main =
   i-am-broken

✔️ PR Todo

• Added/updated unit tests for this change
• Filled out test instructions (In case there aren't any unit tests)
• Included links to related issues/PRs

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