Mention the possibility of top-level errors until testem reports this

closes #2142 

Changed message

> No tests were ran, please ensure that you have a test launcher (e.g. PhantomJS) enabled.

To

> No tests were ran, please check whether any errors occurred in the page (ember test --server) and ensure that you have a test launcher (e.g. PhantomJS) enabled.

I'm working on airportyh/testem#416 to actually let Testem display top level errors in CI mode.