Show an error if a layer is too large.

The map view limits the size of layers (both user and regular) in
order to keep the system responsive, as Leaflet is not great at
handling large layers.

lib/www/client/source/src/views/Map.vue

@@ -475,7 +475,7 @@ export default {
         bounds._northEast.lng,
         bounds._northEast.lat
       ].map(i => i.toFixed(bboxScale)).join(",");
-      const limit = 10000;
+      const limit = 10000; // Empirical value
 
       const query = new URLSearchParams({bbox, limit});
 
@@ -521,7 +521,9 @@ export default {
 
               l.layer.lastRequestURL = url;
             } else {
-              console.warn("Too much data from", url);
+              console.warn(`Too much data from ${url} (${layer.features.length ?? layer.length} ≥ ${limit} features)`);
+
+              this.showSnack([`Layer ‘${l.layer.options.userLayerName}’ is too large: ${layer.features.length ?? layer.length} features; maximum is ${limit}`, "error"]);
             }
           })
           .finally( () => {
@@ -783,7 +785,7 @@ export default {
       this.addUserLayers(userLayers);
     },
 
-    ...mapActions(["api"])
+    ...mapActions(["api", "showSnack"])
 
   },