New way of showing errors based on comments.
I don't want move error up and replace "data", because I want to keep the possibility of having multiple errors. Instead I changed "data" to "errors".
<xml>
<HttpResponse>418</HttpResponse>
<errors>
<error>
<code>AB43</code>
<title>document_title_too_long</title>
<description>The title is limited to 64 characters, yours was 73. </description>
</error>
<error>
<code>AC85</code>
<title>document_no_category</title>
<description>You must select a category to save the document. </description>
</error>
</errors>
</xml>