Proposal: Report which locations a route error is about

While building an app that uses Valhalla I found that a general "failed to route" error left the user experience lacking. On a multi-step route I had to do a human binary search on which location failed.

It would be good to be able to get an index reference to the location or segment that failed the route calculation. That was the UI can match that up with the input location and provide UI affordances (zoom to or visualise the problematic locations/segments).

I have a prototype on forwarding that info into the returned error. But I'm not sure it's an appropriate fix overall. I'll put up a PR for review.

No files or tests are named. Start by tracing the route-calculation error path and reviewing the author's prototype for forwarding the failing location or segment; done means the returned error identifies the relevant input location or segment so a client can match and visualize it.

Written by the indexing model from the issue text.