Add an AlgorithmMismatchError exception

[ixunio]

In preparation for algorithm registry, add an AlgorithmMismatchError exception when 'alg' cannot be processed during decoding.

As seen in:

• JWKError when using a JWKS with multiple algorithms mpdavis/python-jose#138
• Add "algorithm mismatch" error to improve jws mpdavis/python-jose#304