VerifyingKey.verify throws ValueError or AttributeError

[TimDaub]

For some values VerifyingKey.validate throws ValueError or AttributeError, while it should just return False. As far as I remember, this happened when the signature parameter was out of the expected range of the method, so:

1. Inputting a non-string
2. Inputting a non-compliant fulfillment URI ('abc' e.g.)

See how we have to handle this currently in BDB:

• https://github.com/bigchaindb/bigchaindb/pull/641/files#diff-9f1ab5b5daf1b3072e2123b6939bfb07R150
• https://github.com/bigchaindb/bigchaindb/pull/641/files#diff-9f1ab5b5daf1b3072e2123b6939bfb07R171

[diminator]

Is this still an issue with #28 merged?

cc @r-marques