Add error handling for peek function when tokens with invalid payload are passed.

[cosminmodoranu]

This PR contains some changes that add error handling to peeking claims functions.
An issue I encountered is that if we supply a JWT with an invalid payload (like the token below), it would throw an exception.

JOSE.JWT.peek("eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJteSI6IYWltIn0.ePRp64gsJScUPmQ6jJxBnup2ThKMsBeUroa0xopiU")

** (Jason.DecodeError) unexpected byte at position 6: 0x21 ("!")
    (jason 1.4.1) lib/jason.ex:92: Jason.decode!/2
    (jose 1.11.9) src/jwt/jose_jwt.erl:58: :jose_jwt.from_binary/1
    (jose 1.11.9) lib/jose/jwt.ex:193: JOSE.JWT.peek/1
    iex:1: (file)

The changes made in this PR make the functions return an error if the token's payload is malformed (instead of throwing an exception).