Find a way to test for errors reported by Wasp CLI

[sodic]

When releasing 0.18.0, I accidentally used module.exports instead of export default in Open Saas's Tailwind config. Nothing broke in CI because (I'm guessing) this change results only in:

• Broken CSS (handled by Find a way to test for broken CSS #3227 ).
• A logged error under the [Client] prefix.

We should find a way to test for errors reported by the Wasp CLI.

[infomiho]

How would these tests look like? What makes them different from unit tests for an error case?