surface exceptions to MapLibre protocol API [fixes #196]

js/adapters.ts

@@ -123,14 +123,20 @@ export class Protocol {
 
       // TODO: create vector_layers if present to return valid TileJSON
 
-      instance.getHeader().then((h) => {
+      instance
+        .getHeader()
+        .then((h) => {
           const tilejson = {
             tiles: [params.url + "/{z}/{x}/{y}"],
             minzoom: h.minZoom,
             maxzoom: h.maxZoom,
           };
           callback(null, tilejson, null, null);
+        })
+        .catch((e) => {
+          callback(e, null, null, null);
         });
+
       return {
         cancel: () => {},
       };
@@ -181,7 +187,7 @@ export class Protocol {
           })
           .catch((e) => {
             if ((e as Error).name !== "AbortError") {
-              throw e;
+              callback(e, null, null, null);
             }
           });
       });