Error-report if road network is not found

[SannaJokela]

Expected behaviour
When adding GraphHopper url instance, user should know what data is possible for testing. If the point data is out of the road network, users shoud get a warning.

Current behaviour
No warning, isochrone process fails and does not produce any results but also does not give any indicator that things are wrong.

Describe alternatives you've considered
If GraphHopper instance road data is not overlapping the point data ==> user gets a warning: Your GraphHopper instance does not cover this area.

Additional context

[Rikuoja]

Closely related to #7

[Rikuoja]

Now #7 is closed. When no roads were found close to any of the points, the plugin will report an error

 "No results, no roads found close to any of the points: Please make sure that Graphhopper contains the roads in your region."

However, this error only comes after all the points have been checked.

The plugin has no way of knowing what areas the graphhopper instance covers, and graphhopper offers no way to check what the data contains. Therefore, we must check all of the points, because there could be a road next to one of the points.