Add CheckedFromJsonException.badKey (or similar) constructor and field

[kevmoo]

Often when throwing/handling CheckedFromJsonException, you care about the value associated with the key field.

But sometimes, it's the key itself that's invalid – it'd be nice to flag that

CC @jakemac53

[kevmoo]

On second thought, maybe this isn't the place for this - https://github.com/dart-lang/mono_repo/pull/77/files#diff-48eda050bd66ae0f27d3d72d48ea7f9cR10