Make interface routes show proper error messages in incorrect state.

[mizzao]

As described by @kgao in VirtualLab/virtuallab.github.io#4.

This can both cause confusion for developers when testing the app, but also confuse users who press the back button accidentally during a task/experiment.

In considering this approach originally, I thought it would be too heavyweight to automatically change route behavior when adding TurkServer to a Meteor app, but @kgao's experience suggests we may want to reconsider this.