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To provide rich error reporting during manifest pushes, the storage layers verifyManifest stage has been modified to provide the necessary granularity. Along with this comes with a partial shift to explicit error types, which represents a small move in larger refactoring of error handling. Signature methods from libtrust have been added to the various Manifest types to clean up the verification code. A primitive deletion implementation for manifests has been added. It only deletes the manifest file and doesn't attempt to add some of the richer features request, such as layer cleanup.
