doc: make sphinx errors more visible

doc: make sphinx errors more visible

When running Sphinx through ninja, the wrapper configured in meson
redirects stdout to a log file.
It makes more important to print issues on stderr.

Some warnings generated by the conf.py were hidden because
printed on stdout. The first improvement is to print them on stderr.

The second measure is to stop processing if meson was configured
with --werror.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>