do not reload if already loaded or loading

src/ol/DataTile.js

@@ -75,6 +75,10 @@ class DataTile extends Tile {
    * @api
    */
   load() {
+    if (this.state == TileState.ERROR) {
+      this.state = TileState.IDLE;
+    }
+    if (this.state == TileState.IDLE) {
       this.state = TileState.LOADING;
       this.changed();
 
@@ -92,5 +96,6 @@ class DataTile extends Tile {
         });
     }
   }
+}
 
 export default DataTile;