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