Summary:
Throws an exception when:

* The name specified in the manifest mismatches the filename
* Duplicated manifest -- with the sub-directory support it is now able to have multiple manifest files with the same name

Reviewed By: chadaustin

Differential Revision: D17438460

fbshipit-source-id: ac7ad0b701beb15f0e91bb05cd1ec8fe378ad5b6