[universe]: return universe.ErrNoUniverseRoot when no root is found

[guggero]

This is a follow-up issue after fixing #990 in a temporary manner to not break backward compatibility.

Once we shipped the fix for #990 (which is #1316) in v0.5.1 and most clients updated, we can start actually returning a proper error in the QueryAssetRoots RPC if no root is found (the current response is just an empty root).

So this fix can be done for 0.6 or 0.7.