Allow http status code 400 when revoking Vault token accessors

[semiria]

Fixes #34

When revoking a token accessor that has already been revoked/is non existing, Vault sends back http status code 400 which throws an exception and prevents the revoke process to complete on server side. We allow the process to complete by allowing status code 400 in the case of the revoke action.

[VladRassokhin]

Appreciate your help, though I've found solution which also fixes Permission Denied problem which also could arise