Add `rusttype::Error`, for `FontCollection` and `Font` operations.

Created by: jimblandy

This is part of the work for issue #84 (closed), making rusttype handle errors in a more rustic fashion.

• Introduce rusttype::Error, which implements std::error::Error, Debug and Display, and can be converted to std::io::Error.
• Introduce rusttype::Result.
• Use Result to report failure in FontCollection, Font, and associated iterators.