Make build fail if gendoc fails

(cf https://github.com/matrix-org/matrix-doc/issues/1146)

scripts/generate-matrix-org-assets

@@ -2,6 +2,8 @@
 #
 # generate a tarball of assets suitable for the matrix.org site
 
+set -e
+
 cd `dirname $0`/..
 
 mkdir -p assets