Rich error reporting for manifest push

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.

storage/layer.go

@@ -53,9 +53,6 @@ type LayerUpload interface {
 }
 
 var (
-	// ErrLayerUnknown returned when layer cannot be found.
-	ErrLayerUnknown = fmt.Errorf("unknown layer")
-
 	// ErrLayerExists returned when layer already exists
 	ErrLayerExists = fmt.Errorf("layer exists")
 
@@ -65,9 +62,6 @@ var (
 	// ErrLayerUploadUnknown returned when upload is not found.
 	ErrLayerUploadUnknown = fmt.Errorf("layer upload unknown")
 
-	// ErrLayerInvalidDigest returned when tarsum check fails.
-	ErrLayerInvalidDigest = fmt.Errorf("invalid layer digest")
-
 	// ErrLayerInvalidLength returned when length check fails.
 	ErrLayerInvalidLength = fmt.Errorf("invalid layer length")
 
@@ -75,3 +69,21 @@ var (
 	// Layer or LayerUpload.
 	ErrLayerClosed = fmt.Errorf("layer closed")
 )
+
+// ErrUnknownLayer returned when layer cannot be found.
+type ErrUnknownLayer struct {
+	FSLayer FSLayer
+}
+
+func (err ErrUnknownLayer) Error() string {
+	return fmt.Sprintf("unknown layer %v", err.FSLayer.BlobSum)
+}
+
+// ErrLayerInvalidDigest returned when tarsum check fails.
+type ErrLayerInvalidDigest struct {
+	FSLayer FSLayer
+}
+
+func (err ErrLayerInvalidDigest) Error() string {
+	return fmt.Sprintf("invalid digest for referenced layer: %v", err.FSLayer.BlobSum)
+}