Modify mockAPI to return correct answer for not existing files

This change fixes couple of wt2wt tests, because what previously was
parsed as <figure> is now handled as <meta> placeholder so there is no 
information lost while serializing.

Change-Id: Iec1ec2f099dd6d850d70aa4e48e813cd4ade07c7
Merge "Modify mockAPI to return correct answer for not existing files"