Enable nested namespacing to be shown in MockProver::assert_satisfied output

[str4d]

MockProver::assert_satisfied prints out the names of regions in its error output, which is nice. However, if a developer is using a third-party chip in their circuit, they may not know what its region names are, and have a harder time identifying where the error comes from.

We should do whatever we need to do in order to enable regions created by chips to be identified as such in error output. bellman would track nested namespaces; it might be possible to do something similar in halo2_proofs.