be more verbose about source of tile errors

ios/MBXViewController.mm

@@ -473,6 +473,8 @@ namespace llmr
                 {
                     res.code = [(NSHTTPURLResponse *)response statusCode];
                     res.body = { (const char *)[data bytes], [data length] };
+                } else {
+                    NSLog(@"http error (%s): %@", url.c_str(), [error localizedDescription]);
                 }
 
                 background_function(res);

src/map/tile_data.cpp

@@ -58,7 +58,7 @@ void TileData::request() {
             tile->data.swap(res.body);
             tile->parse();
         } else {
-            fprintf(stderr, "tile loading failed\n");
+            fprintf(stderr, "tile loading failed: code %d\n", res.code);
         }
     }, []() {
         platform::restart();