RenameNamespace RPC creates parent directories for the new location

By debugging why the pipeline failed on a CE MR, it appeared that the behaviour of the RenameNamespace RPC was inconsistent with other RPCs that require a parent directory. This commit fixes that behaviour so the CE MR can move along afterwards.

Futher, this commit includes minor changes to the namespace service to clean it up a bit. Not strictly required, but personnally it always bugged me a bit that RPCs called others RPCs. This service was the first, or second RPC that I built on this project, so I felt it was nice to clean up the mess past me left behind.

The helper package now gets two more convenience methods to remove the need for helper.ErrInternal(fmt.Errorf("string %v", err)).