Tests: fix ignored return value from htmlproofer

Fixes bug in the tests introduced in
0e7d61a

Author: harding

Makefile

@@ -12,7 +12,7 @@ test: test-fast test-slow
 
 test-slow:
 	## Check for malformed HTML and broken internal links
-	bundle exec htmlproof --check-html --disable-external --url-ignore '/^\/bin/.*/' ./_site | cat
+	bash -c "set -o pipefail ; bundle exec htmlproof --check-html --disable-external --url-ignore '/^\/bin/.*/' ./_site | cat"
 	## Check that links on the /en/download page point to the separately-stored binaries in /bin
 	contrib/qa/test-binary-availability.sh && echo "SUCCESS checking URLs for binaries"