Fix error handling in DeleteFileBulk storage handler (#8327)

errors.errorString() cannot be marshalled by gob
encoder, so using a slice of []error would fail
to be encoded. This leads to no errors being
generated instead gob.Decoder on the storage-client
would see an io.EOF

To avoid such bugs introduce a typed error for
handling such translations and register this type
for gob encoding support.

cmd/server-main.go

@@ -37,6 +37,7 @@ func init() {
 	logger.Init(GOPATH, GOROOT)
 	logger.RegisterUIError(fmtError)
 	gob.Register(HashMismatchError{})
+	gob.Register(DeleteFileError(""))
 }
 
 // ServerFlags - server command specific flags