show error when running iOS app tests and the TEST_HOST doesn't exist.

We'll now show...

```
  run-test TestProject-LibraryTests2.octest (iphonesimulator6.1,
application-test, GC OFF)
  failed: TEST_HOST doesn't exist at '/Users/fpotter/Library/Developer/Xcode/DerivedData/TestWorkspace-Library-dfkuuumeqgksbpctikbmroulmipg/Build/Products/Debug-iphonesimulator/TestProject-TestHost.app'
```