Properly fail in travis if sub-script fail.

Make `kiwix-build.py` also fail if sub-command fail instead of just print
an error message and quit.

`kiwix-deploy.py` is far more simple and already fails miserably if a
sub-command fails.

travis/install_extra_deps.sh

@@ -1,5 +1,7 @@
 #!/usr/bin/env bash
 
+set -e
+
 orig_dir=$(pwd)
 
 sudo apt-get update -qq