Make sure processResponsiveImages checks for valid thumb object

If the thumbnail is an error (or worse yet, if the MediaHandler
returns false, like TMH does in certain circumstances), we
should not add responsive thumbnail links.

Change-Id: I2aafbd07e8435ce2d1b4cb32e49e98cb5f6c00ab
Merge "Make sure processResponsiveImages checks for valid thumb object"