Warn or error on invalid extras

[eaubin]

If an extra is misspelled or missing, tox should give an error or warning. For example when running tox -e badextras with

[testenv:badextras]
extras = missing
commands = python --version

I came across this when I was debugging a project with an env like

[testenv:docs]
...
extras = build_docs

where the project.optional-dependencies were not getting installed (the underscore is problematic https://github.com/tox-dev/tox/issues/2655). A warning or error would have been very helpful figuring out what was happening.