Return the number of errors we saw, just like a lint or a test script.
Return the number of errors we saw, just like a lint or a test script.

This will be useful for using this script to test whether a component
is server-side renderable or not.

Auditors: jlfwong

Test plan:
   % tools/benchmark_ka_component.js
   javascript/content-library-package/components/concept-thumbnail.jsx: 1679 275ms
   DONE!
   0

   % tools/benchmark_ka_component.js ~/khan/webapp/javascript/content-library-package/tutorial-nav.jsx; echo $?
   Skipping /home/csilvers/khan/webapp/javascript/content-library-package/tutorial-nav.jsx: cannot infer component from /home/csilvers/khan/webapp/javascript/content-library-package/tutorial-nav.jsx
   DONE!
   1

First one had 0 errors, the second had 1.

Khan/react-render-server,Khan/react-render-server,Khan/react-render-server