fix more AssertionError's for new-style steps in web status ui

This fixes the following urls:
  http://localhost:8010/builders/<builder name>/builds/<build no>/steps/<step name>/logs/<log name>
  http://localhost:8010/builders/<builder name>/builds/<build no>/steps/<step name>
  http://localhost:8010/waterfall
Merge pull request #1275 from thedylman/fix_new_style_steps_in_web

fix more AssertionError's for new-style steps in web status ui