res_pjsip_stir_shaken: RFC 8225 compliance and error message cleanup.

From RFC 8225 Section 5.2.1:

    The "dest" claim is a JSON object with the claim name of "dest"
    and MUST have at least one identity claim object.  The "dest"
    claim value is an array containing one or more identity claim JSON
    objects representing the destination identities of any type
    (currently "tn" or "uri").  If the "dest" claim value array
    contains both "tn" and "uri" claim names, the JSON object should
    list the "tn" array first and the "uri" array second.  Within the
    "tn" and "uri" arrays, the identity strings should be put in
    lexicographical order, including the scheme-specific portion of
    the URI characters.

Additionally, make it clear that there was a failure to sign the JWT
payload and not necessarily a memory allocation failure.

Change-Id: Ia8733b861aef6edfaa9c2136e97b447a01578dc9