Use a more obvious placeholder image in iOS app instead of the app icon (#1636)

platform/ios/app/Assets.xcassets/MissingImage.imageset/Contents.json

@@ -0,0 +1,21 @@
+{
+  "images" : [
+    {
+      "idiom" : "universal",
+      "scale" : "1x"
+    },
+    {
+      "filename" : "missing.png",
+      "idiom" : "universal",
+      "scale" : "2x"
+    },
+    {
+      "idiom" : "universal",
+      "scale" : "3x"
+    }
+  ],
+  "info" : {
+    "author" : "xcode",
+    "version" : 1
+  }
+}

platform/ios/app/MBXViewController.m

@@ -1807,7 +1807,7 @@ CLLocationCoordinate2D randomWorldCoordinate(void) {
 }
 
 - (UIImage *)mapView:(MLNMapView *)mapView didFailToLoadImage:(NSString *)imageName {
-    UIImage *backupImage = [UIImage imageNamed:@"AppIcon"];
+    UIImage *backupImage = [UIImage imageNamed:@"MissingImage"];
     return backupImage;
 }