Warn about invalid extras during locking

[uranusjr]

Not sure if this is already done. pypa/pipenv#3026.

[techalchemy]

Likewise, but I don’t think we should fail... presumably we shouldn’t write the bad value?

[uranusjr]

I was thinking about dropping invalid extras as well, but that would be more involved to implement. Nice to have, but not necessary and don’t need to be done right now.