Return a helpful error message when login fails

[btipling]

When the user did not setup the box app correctly or used invalid credentials the Box API returns a helpful error message but we are not showing it to the user.

connector","error":"box-connector: failed to get token: failed to get access token: {\"error\":\"invalid_client\",\"error_description\":\"The client credentials are invalid\"} status: 400 Bad Request"}
box-connector: failed to get token: failed to get access token: {"error":"invalid_client","error_description":"The client credentials are invalid"} status: 400 Bad Request

Before it wouldn't error at all, it assume authentication succeeded and would try to authenticate with an empty string access token and fail later with an unhelpful error.