Giva a better error message than 'Transform output ... must exist'

[jjohannes]

For context see #78 and #104.

If we can detect that Gradle will eventually fail with this error, we could give a more actionable advice instead.

[jjohannes]

Related Gradle issue: https://github.com/gradle/gradle/issues/26155

[jjohannes]

Another related issue: https://github.com/gradle/gradle/issues/24064

[jjohannes]

Instead of somehow modifying the error, we should think about a sustainable solution: #161