Add JsonErrorMixin:

This commit adds the JsonErrorMixin for formatting error message from a
given RequestHandler as JSON.
Fix typo in README.
Clean up write_error.

This commit alters write_error to pull the message from a
get_message function of the error, else it using a default Unexpected
Error string.
write_error: get exc_info should return a triple.
Merge pull request #1 from sprockets/write_error

Add JsonErrorMixin