Ignore validation servers returning YKCLIENT_BACKEND_ERROR

[alexjfisher]

This will happen if mysqld has failed on the validation server. Previously ykclient lib would return YKCLIENT_BAD_SERVER_SIGNATURE without waiting for other servers to return.

I've tested this and it fixes https://github.com/Yubico/yubico-c-client/issues/27

[jas4711]

Thanks, we've discussed this a bit and this definitely should be fixed. This may indicate some more serious issue with the client algorithm so we were thinking of writing a document describing that, and then checking that the code matches that.