Add notification when isochrones cannot be calculated

[rmglennon]

If isochrones cannot be generated from the pin's position, add a notification that a problem has occurred. See also #265

To reproduce:

• Pan to the ocean...or anywhere else known without roads or transit data.
• Drag a pin onto the map.
• Click Generate isochrones.

The map refreshed but it looks like nothing has happened. There is a console error of 400, and the response is:

{"error_code":171,"error":"No suitable edges near location","status_code":400,"status":"Bad Request"}

Maybe something like, "Isochrones could not be generated for this area. Make sure you are near roads or transit network lines and try again."