Add more links to [ErrorMessage]s throughout the application

[edsko]

(particularly with authentication)

[dcoutts]

@gracenotes could you enlighten us as to what this TODO item meant?

[gracenotes]

This is based on the semi-RESTful idea that people should not reach dead ends, that is, pages with only error messages where nothing that can be done. If there is no package by a name, show fuzzy-matching suggestions; if auth failed, say why and link to necessary resources. This is about adding MLinks within error response to make sure clients have a place to go.