Better error message required for `toFile`

[marshallswain]

I ran the following code and service was undefined, which caused an error:

toFile(({ lib = 'src', service, files }: Context) => [
  lib,
  'services',
  service,
  files.main,
]),

but it was way too hard to track this down with only this much information about the error in the console:

? What is the dashed name of your service? (e.g. category-libraries) folders
Oh no! Something went wrong: The "paths[3]" argument must be of type string. Received undefined

We need a better message and a stack trace. I couldn't find the cause of the error in the debugger, either.