Fix response.exists() logic

[dmitrizagidulin]

Currently, response.exists() will never return false (since the web.solidRequest() method returns an error object when status < 300). Fix this.

[elf-pavlik]

same applies to solidResponse.xhr.status === 403 suggested in https://github.com/solid/solid-client#getting-information-about-a-resource

responsible code in solid-web-client : https://github.com/solid/solid-web-client/blob/f7c9f2e673bc956c67fcee091beaeebfc35687d9/src/client.js#L312-L318