Check the keychain for the token before falling back to ATOM_ACCESS_TOKEN.
Check the keychain for the token before falling back to ATOM_ACCESS_TOKEN.

Otherwise auth failures get confusing: we’d keep updating the token
stored in the keychain when the real problem was that ATOM_ACCESS_TOKEN
was out of date. See #505.

Nikpolik/apm,bronson/apm,Nikpolik/apm,atom/apm,bronson/apm,atom/apm,atom/apm,bronson/apm,atom/apm,bronson/apm,Nikpolik/apm,Nikpolik/apm