we refactored `userIDFromToken` for the token parsing part into a new
function `parseToken`. `parseToken` returns the string `token` from
request, and a boolean `ok` representing whether the token exists or
not. So we can distinguish between token non-existence and token
inconsistency in the `verfity` function, thus solving the problem of no
proper error message when the token is inconsistent.
close go-gitea#24439  
related go-gitea#22119

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Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: Giteabot <teabot@gitea.io>