Code errors should break the build

Invalid JS, like adding an extra : to your action code will cause `aio app build` to choke and output a verbose error message that a developer can use to fix their code. ✅

However, if I call a function that is not import/required or reassign a value to a const it will happily webpack it .. even though, come runtime, it will certainly crash ❌

To test, simply add this to an action and watch it happily build ...
```
const num = 100
num = num + 1
```

Start by reproducing the issue in an action with the shown const reassignment and run `aio app build`; compare this with the existing verbose syntax-error behavior. Trace the build validation or webpack entry point responsible for accepting the code. Done means runtime-invalid JavaScript causes the build to fail with an actionable error.

Written by the indexing model from the issue text.