Improve error message in the TestxDirector

[b1zzu]

Steps to reproduce

1. Create a TestxDirector with a wrong TestxController url

   const director = new TestxDirector("http://some.url:5555")
   await director.start()

   it will throw an error similar to this

   TypeError: Cannot read property 'data' of undefined
   at TestxDirector.eval (webpack:///./node_modules/graphql-testx/dist/src/TestxDirector.js?:27:60)
   at Generator.throw (<anonymous>)
   at rejected (webpack:///./node_modules/graphql-testx/dist/src/TestxDirector.js?:6:65)

   Expected result

   I would like to se a more explicity error, like the TestxController can not be reached on http://some.url:5555