Malformed JSON throws undocumented DomainException

[markrandall]

The decode method documents the following list of exceptions to decode:

* @throws InvalidArgumentException     Provided JWT was empty
     * @throws UnexpectedValueException     Provided JWT was invalid
     * @throws SignatureInvalidException    Provided JWT was invalid because the signature verification failed
     * @throws BeforeValidException         Provided JWT is trying to be used before it's eligible as defined by 'nbf'
     * @throws BeforeValidException         Provided JWT is trying to be used before it's been created as defined by 'iat'
     * @throws ExpiredException             Provided JWT has since expired, as defined by the 'exp' claim

However, this is incomplete. Providing a malformed payload results in an undocumented DomainException which propagates up the stack from self::jsonDecode

<?php
 \Firebase\JWT\JWT::decode('thisobviouslyisnt.validbase64.encodedjson', '123');

=> DomainException

[markrandall]

Duplicate of #312