WMS server: throw LayerNotDefined exception when undefined layer is requested

This is according to WMS 1.1.0 / 1.1.1 / 1.3.0 specs

Author: wonder-sk

src/server/qgswmsprojectparser.cpp

@@ -199,6 +199,9 @@ QList<QgsMapLayer*> QgsWMSProjectParser::mapLayerFromStyle( const QString& lName
     }
   }
 
+  if ( layers.count() == 0 )
+    throw QgsMapServiceException( "LayerNotDefined", QString( "Layer \"%1\" does not exist" ).arg( lName ) );
+
   return layers.values();
 }