Its a dynamic error, if @content-type is not valid..
Test ab-load-009.xml is expected to fail with error code err:XD0079 [Core steps, XProc]
.
<p:declare-step xmlns:err="http://www.w3.org/ns/xproc-error" xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
<p:output port="result"/>
<p:load content-type="surely-not-correct" href="file.xml"/>
</p:declare-step>
Additional tests for p:load.