doc: Fail when downloading images from Debian wiki fails

Fixes: #2173.

When a wget process fails to download an image from Debian wiki for any reason,
fail the whole fetching process so that the users notices it and reruns the
process. Earlier, these were failing silently leading to zero sized images.

Tests:

- Without the change, notice that when wget process is killed during fetching of
images, it leads to incorrect image files (partially downloaded or zero sized
files).

- With or without the changes, killing wget process at other times leads to
fetch process halting indicating that wget in normal cases fails correctly.

- With the change, killing the wget process will lead to an error and the
fetching process stops.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

doc/Makefile

@@ -85,7 +85,7 @@ $(fetch-main-list): fetch-main-%: fetch-cleanup-%
 	xsltproc $(SCRIPTS_DIR)/fetch-images.xslt manual/$(lang)/freedombox-manual.raw.xml | \
 	  sort -u | \
 	  awk 'NF {print "wget --quiet -O manual/$(lang)/images/" $$1 " " $$2}' | \
-	  sh
+	  sh -e
 	rm manual/$(lang)/freedombox-manual.raw.xml
 
 fetch-pages-%: lang = $*