Improve UX when a resource mount cannot be found

- Add a full example of the docs. Hopefully is a little more explicit.
 - Print a warning when the mount is empty.

Closes #762

src/main/java/dan200/computercraft/api/ComputerCraftAPI.java

@@ -98,7 +98,9 @@ public static IWritableMount createSaveDirMount( @Nonnull World world, @Nonnull
      * resource folder onto a computer's file system.
      *
      * The files in this mount will be a combination of files in all mod jar, and data packs that contain
-     * resources with the same domain and path.
+     * resources with the same domain and path. For instance, ComputerCraft's resources are stored in
+     * "/data/computercraft/lua/rom". We construct a mount for that with
+     * {@code createResourceMount("computercraft", "lua/rom")}.
      *
      * @param domain  The domain under which to look for resources. eg: "mymod".
      * @param subPath The subPath under which to look for resources. eg: "lua/myfiles".

src/main/java/dan200/computercraft/core/filesystem/ResourceMount.java

@@ -103,9 +103,13 @@ private ResourceMount( String namespace, String subPath, IReloadableResourceMana
     private void load()
     {
         boolean hasAny = false;
+        String existingNamespace = null;
+
         FileEntry newRoot = new FileEntry( new ResourceLocation( namespace, subPath ) );
         for( ResourceLocation file : manager.listResources( subPath, s -> true ) )
         {
+            existingNamespace = file.getNamespace();
+
             if( !file.getNamespace().equals( namespace ) ) continue;
 
             String localPath = FileSystem.toLocal( file.getPath(), subPath );
@@ -114,6 +118,15 @@ private void load()
         }
 
         root = hasAny ? newRoot : null;
+
+        if( !hasAny )
+        {
+            ComputerCraft.log.warn("Cannot find any files under /data/{}/{} for resource mount.", namespace, subPath);
+            if( newRoot != null )
+            {
+                ComputerCraft.log.warn("There are files under /data/{}/{} though. Did you get the wrong namespace?", existingNamespace, subPath);
+            }
+        }
     }
 
     private FileEntry get( String path )