Assorted fixes

The most important fix is the one changing the signature files' names to appropriate ones. It actually would merit an increment of the major version according to [SemVer](http://semver.org/).

The other important change is to give a more helpful message to the user than a `NullPointerException` when BouncyCastle fails to handle an RSA (sign-only) key properly (it *can* handle DSA (sign-only) keys just fine...).