Improve error messages

[koebi]

Here's what I did

Request something that leads to an error, like #1039 did (both in the original request and in my example posted below).

Here's what I got

Unhelpful error message 6099: Unknown error or 2009: Cannot find route

Here's what I think could be improved

Helpful error messages - in the case of 6099, something along the lines of Some routes could not be calculated., possibly with an info on which point-to-point-connection is the issue.