Fix output buffer fatal error due to not properly handling class method output callbacks

[westonruter]

When activating the all-in-one-seo-pack plugin, an error shows up in the error log when validating a URL:

PHP Fatal error: ob_start(): Cannot use output buffering in output buffering display handlers

The underlying code in the plugin that causes the problem is the adding of the All_in_One_SEO_Pack ::output_callback_for_title() method as an output buffer callback in:

https://github.com/semperfiwebdesign/all-in-one-seo-pack/blob/982cbb6a6ba183cf538d86b858273505a417720f/aioseop_class.php#L3496

The AMP plugin is supposed to prevent this error from happening via the AMP_Validation_Manager::can_output_buffer() method. It turns out there was a flaw in the logic being used to convert a function on the call stack into the format expected by ob_list_handlers(). In particular, it was not handling the case where an output callback was a class method. The issue is fixed by properly detecting when a call stack entry is for a class method.

This pull request also improves error message shown when a validate_url call fails. Instead of showing an error code like response_comment_absent it will now show a full message explaining what is going on. This change also adds WSOD (whitescreen of death detection) as a more specific error case for when the JSON comment is absent:

[amedina]

Ship it.

[postphotos]

This one was fun to find. 🐞 Thanks for digging in, @westonruter!