Previously only the line numbers of a giant, internally generated XML file were
printed, without any kind of debuggability.
Now at least the mentioned lines are printed with a little bit of surrounding
context (to have something to grep for).
```
manual-combined.xml:4863: element para: Relax-NG validity error : Did not expect element para there
4859 <chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xi="http://www.w3.org/2001/XInclude" version="5.0" xml:id="sec-writing-modules">
4860
4861 <title>Writing NixOS Modules</title>
4862
4863 <para>NixOS has a modular system for declarative configuration. This
4864 system combines multiple <emphasis>modules</emphasis> to produce the
4865 full system configuration. One of the modules that constitute the
```
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