Catch Registry Errors

This is part of [this ticket](https://www.pivotaltracker.com/story/show/80866418), though we still need to finish the UI side of things before that is finished. This will allow us to loop over all of the registries and return some mixture of results and detailed errors, depending on how the various registries respond to the search.

Right now we've just got a blanket rescue around the search request, but it will be possible to return more specific messages to various exceptions in the future (like a *You need auth but we don't support it* message when we get a 403 response).