`vamp-ir halo2 verify` exits with status 0 for invalid proofs

[tkerber]

I'd expect invalid proofs to output a non-zero status code to indicate an error. At least the halo2 backend will print the failure, but exit as if verification had been a success.

[lopeetall]

Good catch. We are also working to improve error reporting #80 and this dovetails nicely.