Better error chaining

We chain error messages coming from Neptulon to Titan client and going back through `ctx.Next()` so log prefixes for errors gets mixed. Especially `titan/client/route_out.go` suffers from that. We need a better approach than just prefixing error messages (time for structured logging?).

Ref: https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=golang%20error%20chaining
