[Tracking] Add error handling

[wschella]

To make a robust and deploy-ready app, the current code needs at least some error handling, I think providing handlers for following logic provide the most value:

• mu-search server calls. The server can be down or slow, and we could have introduced a bug w.r.t. query parameters. The application can't quite continue in this case, as there are no search results to render.
• Rendering a (single) search result. In most cases an error in rendering a search result would also mean errors in the other search results, but there are plenty of possibilities it's because of an unexpected attribute value in a single result (e.g. list where a single value was expected, a missing field, ...), and this way you could still display partial results.
• Catch-all application error. We should at least display something nice & a link to the issue list. This would also catch errors to mu-resource.