[traffic] Improve an error msg

simtools.gaml.extensions.traffic/src/simtools/gaml/extensions/traffic/driving/DrivingSkill.java

@@ -1266,7 +1266,7 @@ public IPath primComputePath(final IScope scope) throws GamaRuntimeException {
 			}
 			path = PathFactory.newInstance(graph, source, target, edges);
 		} else {
-			throw GamaRuntimeException.error("either `nodes` or `target` must be specified", scope);
+			throw GamaRuntimeException.error("one of `nodes` or `target` must be non nil", scope);
 		}
 
 		// restore graph direction