nixos/doc/manual: print context on failing xmllint validation

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
```