Change how verifyJWT handles errors

[theS1LV3R]

Currently if verifying the JWT fails at all it returns a nondescript error. This should be changed, so it throws a specific error depending on what failed (time expired, invalid signature, etc).

• https://github.com/auth0/node-jsonwebtoken/blob/master/verify.js - Underlying command
• https://github.com/auth0/node-jws/blob/master/lib/verify-stream.js - Errors