Expand exception management in JWHelper implementations for comprehensive error messages

[Zicchio]

When decrypting or verifying a token, it might be useful to dedicate specific exception for malformed input (ex not a jwt) vs an invalid token (non verified signature, expired token, exc). This is both at the code level AND in the docstring methods.

This is not a strong requirement, but would be eventually nice to have so that HTTP Handlers using the token verifier / token decrypter can yield meaningful and informative error messages (with the proper concern for minimal information disclosure), which is currently somewhat hard to do.