fix(waitforangular): improve error messages when waitForAngular fails

Previously, caught errors were being interpreted as an empty object, causing
lots of errors such as
'Uncaught exception: Error while waiting for Protractor to sync with the page: {}'
Now the error message will be displayed, and a more useful custom message
will be thrown if the variable 'angular' is not present or the root element
is not part of the ng-app.

See #1474