Exception handling on Key.verify for incompatible type of signature

[PradyumnaKrishna]

Description of issue or feature request:
Key.verify method can produce unknown error's during verification of signatures. Example: If a GPGKey is used to verify a signature of type Signature, will cause a FormatError. With FormatError determining the cause of error is not easy.
Key.verify method should handle these exceptions and raise exceptions for different causes.

Current behavior:
No error handling in Key.verify.

Expected behavior:
Error handling should be added to Key.verify.

Ref: #2 (comment)

[lukpueh]

TLDR #2 (comment): Change Key.verify to return None on success and raise error otherwise.

[lukpueh]

Fixed upstream in secure-systems-lab#456, see signer.Key.verify_signature.