Add MessagePack serialization of HTTPError instances

[kgriffs]

but... how to make the msgpack library an optional dependency?

[rouge8]

but... how to make the msgpack library an optional dependency?

setuptools supports extras_require to declare optional dependencies, which can be installed like pip install falcon[extra-feature-name]. It's a bit annoying in tox though, the easiest way I've found is setting skipsdist = true and then adding -e{toxinidir}[feature1,feature2] to the deps section of the testenv.

[kgriffs]

Ah, good to know. Thanks!

[vytas7]

We don't want to special case MessagePack, but instead deliver the same functionality as part of #2023.