Check's error has path in the object tree.
Nick's comments. One line Error message, comments.
Improve comment. Cleaner way to construct path string.
Remove duplication in error of unknown key.

"Unknown key in field Y" instead of "Unknown key Y in field Y"
Add Match.Integer match for 32-bit integers. #1171
Docs for Match.Integer
Make docs for Match.Optional more truthy about undefined in an object.
Nick's comments.

Don't duplicate the logic.
Grammar corrections. Comments moving.
Better docs for Match.Optional.
Merge branch 'check-path' into devel

Small improvements to 'check' package:

- Match.Integer matches 32-bit integers.
- Match.Error shows full path of sub-object where error occurred.
- Better docs for Match.Optional.