Add feature to fill null table entries with as-the-crow-flies estimates.

src/server/service/table_service.cpp

@@ -56,6 +56,11 @@ std::string getWrongOptionHelp(const engine::api::TableParameters &parameters)
         help = "Number of coordinates needs to be at least two.";
     }
 
+    if (parameters.fallback_speed < 0)
+    {
+        help = "fallback_speed must be > 0";
+    }
+
     return help;
 }
 } // anon. ns