Comment 17 for bug 1098615

If you don't have the hud package installed, you get:
  Error: schema com.canonical.indicator.appmenu.hud not installed

If you don't have unity-webapps-service, you get:
  Error: schema com.canonical.unity.webapps not installed

In both of this cases, you can't open unity-tweak-tool at all. These package should either be a dependency of unity-tweak-tool, or the tool made to work and only give warnings if they are not present. The current (in Trusty) functionality is not acceptable.