Report error details when Cesium ion token exchange fails

Currently, if Cesium ion returns an error code when we try to complete token exchange during sign-in in the `CesiumIonClient::Connection` class, we report to the user (via an HTML page returned by the local server) that Cesium ion returned an error with a particular code. But we don't include the error message or any other response content, which can sometimes make it difficult for users to know what has gone wrong.

When Cesium ion returns an error in its standard format (an example of which can be seen by simply visiting https://api.cesium.com/), we should report the message to the user. In other cases, we should display the entire response content, maybe in an iframe or something.

Start in the CesiumIonClient::Connection token-exchange path and follow how the local server builds the HTML error page. Exercise failures with Cesium ion’s standard error format and with other response content. Done means the page reports the standard error message or displays the full response content instead of only the error code.

Written by the indexing model from the issue text.