Insufficiently specific POST request validation on `/api/route`

[davidwyld]

I accidentally sent a malformed POST request from the swagger ui (trailing comma, gasp) and as a result get the error message "Invalid coordinate values provided: start_lat" - which isn't really the problem. We could do with refining the input validation logic here.

https://github.com/bas-amop/PolarRoute-server/blob/c670363e737abfe78824eb29f0586ecc55650382/polarrouteserver/route_api/views.py#L339-L347