Fix #99.
Fix #99.

Improve failure mode for bad struct field name mapping.

zsiciarz/docopt.rs,brandonson/docopt.rs,birkenfeld/docopt.rs,zsiciarz/docopt.rs,birkenfeld/docopt.rs,zsiciarz/docopt.rs,brandonson/docopt.rs,birkenfeld/docopt.rs,brandonson/docopt.rs