Allow errors in app.transaction

[neokry]

Currently app.transaction requires a TransactionResponse return value. If a request fails validation ie if input text is below a certain value or if a query string is invalid an error response should be returned instead of a broken TransactionResponse.

[jxom]

You can now return Response objects from 0.3.1:

app.transaction('/tx', c => {
  return new Response('dang', { status: 400 })
})