change error message

[kurehajime]

change invalid token error message.
because invalid token error is output even if network error.

[before]
if invalid token and online.

$./slack-dump -t=INVALID_TOKEN
->ERROR: the token you used is not valid...

if valid token and offline.

$./slack-dump -t=VALID_TOKEN
->ERROR: the token you used is not valid...

[after]
if invalid token and online.

$./slack-dump -t=INVALID_TOKEN
->ERROR: Authentication failed.(invalid_auth)

if valid token and offline.

$./slack-dump -t=VALID_TOKEN
->ERROR: Authentication failed.(Post https://slack.com/api/auth.test: dial tcp: lookup slack.com: no such host)

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無効なトークンによるエラーとネットワークエラーの切り分けができればありがたいな、という趣旨のプルリクエストです。