<p>Well, having machine readable error codes isn't a concept that is widely available in the API as of today. This leads to all sorts of regular expression based error message parsing, going back to even Potlatch2: <a href="https://github.com/openstreetmap/potlatch2/blob/master/net/systemeD/halcyon/connection/XMLConnection.as#L330-L364">https://github.com/openstreetmap/potlatch2/blob/master/net/systemeD/halcyon/connection/XMLConnection.as#L330-L364</a></p>
<p>I certainly wouldn't object putting in some more accessible error code in the "Error" HTTP header, and have the localized text in the body for the UI to show in a popup. After all, JOSM just needs the Error header to reliably recognize a "User has been blocked" situation.</p>

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