WIP - Detected failed test run

closes #2142 

I'm working on this but rather than just changing the message I'ld like to actually display the error('s) that occurred (if any). As you can see below, the case I'm testing for is a syntax error which isn't caught at build time (a malformed JSON string). 

I'm running into a bit of an issue because testem doesn't report this as an "error" (but rather just as 0 tests being run). I'm guessing this is because the error occurs before the testing code started..  In https://github.com/airportyh/testem/blob/master/assets/phantom.js I see that there's also no 'error' hook attached to the page in phantom so unless I'm missing something, I guess I'll have to open a PR with testem? Can you confirm this @stefanpenner  or @rwjblue ?

Also: If that's the case, would you still like a PR where I append something like "Make sure that no errors occurred by running the tests in you browser with ember test --server" to the "No tests were run" eror?