Dump server exception onto stderr
Dump server exception onto stderr

I had a silent failure when there was a missing asset file, showing
only #<Selenium::WebDriver::Error::JavascriptError: unknown error>. Now
it prints something like "ActionView::Template::Error: couldn't find
file 'ember'" followed by a lengthy stack trace.

hugocorbucci/konacha,jfirebaugh/konacha,hugocorbucci/konacha,uken/konacha,uken/konacha,jfirebaugh/konacha,jfirebaugh/konacha,uken/konacha,hugocorbucci/konacha