cdk migrate cli UX issue

[SimonCMoore]

When not supplying stack name an error with unclear messaging is shown. Expectation state the missing parameters

$ ./node_modules/cdk/bin/cdk migrate --from-path template.yaml  --output-path cdkstack/
  ❌  Migrate failed for undefined: stack generation failed due to error 
'The text and separator arguments should be of type string'

supplying --stack-name test as well resolves the issue but it was unclear that this was needed from the error.