Permission denied error could be handled better

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Vault returns status 400 with body {"errors":["permission denied"]} if access token is incorrect or has insufficient permissions.

On receiving such status, vault-java-driver throws com.bettercloud.vault.VaultException: Expecting HTTP status 204 or 200, but instead receiving 400.
Which is very confusing, because Vault documentation says that it means 400 - Invalid request, missing or invalid data. See the "validation" section for more details on the error response.
And it doesn't provide any more details.

vault-java-driver makes it hard to debug by not exposing response body.
I think that vault-java-driver should incorporate contents of errors field in exception message.

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Fun fact - it probably isn't a permission issue.
My token works in the REST client, but when I use it with BetterCloud vault-java-driver, I get status 400.
I tried debugging it and evaluated expression in this line: https://github.com/BetterCloud/vault-java-driver/blob/master/src/main/java/com/bettercloud/vault/api/Logical.java#L129

final int restStatus = restResponse.getStatus();

Expression new String(restResponse.getBody(), "UTF-8") returns empty string.

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I apologize for spamming.

I made an identical request using invalid token and received {"errors":["permission denied"]} in REST client, but vault-driver still shows that response body is empty so it is impossible to debug it.