Follow-up to f18362ccce.
While working on I625a48a6ecd3fad5c2ed76b23343a0fef91e1b83,
I learned that Stringable objects are sometimes also used
as normal params, intended to be stringified when rendering.
For example, see `this->target` in the UnblockUser class.
That code uses Message objects, so it's not causing trouble,
but converting them to MessageValue would emit warnings.
I didn't actually intend to deprecate that, and it won't
cause issues with JSON-serializability (assuming we stringify
the objects in time), so tweak how the warnings are emitted.
Change-Id: I26dfd4f1ac8ed08a422692de4e39d072242c08df
7ad5a2ca54