Set indentWidth=2, to match the existing style.
Fix use of self inside blocks.

This will generally cause retain cycles.
Don't pass an empty block to imageAtURL.

imageAtURL checks for NULL, so there's no need to allocate and call a
block that does nothing.  We can just pass NULL instead.
Factor multiple references to [self stringEncoding] to a local variable.

This removes the reference to self inside the blocks.
Remove multiple references to self inside blocks.

This avoids the risk of any retain cycles being created.
Log an error if it occurs.
Add a sanity check that unarchiveObjectWithFile is actually
giving the MKNetworkOperation that we expect.