Invalid parameters

Backbone route parameters are decoded via `decodeURIComponent`. If the parameter is malformed (e.g. `%foo`) then a `URIError` is thrown. Since the user is in control of the url, this can happen even if all caution is taken to encode parameters. Is this truly an exceptional case in which things are so broken that execution should stop? If not, what should we return as a parameter?

The issue names Backbone route parameters and decodeURIComponent but no file or test. Start by locating the route-parameter decoding path and reproducing a malformed value such as `%foo`; done requires an agreed behavior for that parameter and coverage showing the chosen behavior without breaking valid encoded parameters.

Written by the indexing model from the issue text.