Validation bugfixes and error handling

* Fix bug in verify_hashcash to search db for the token, not the digest.
* Streamline if logic in show_post.
* Add error reporting for missing/invalid hashcash token.
* Add error reporting for missing passphrase when duress is enabled.
* Add template, js, css necessary to display errors.
Merge pull request #25 from fungus/verify_hashcash

Validation bugfixes and error handling