docs/provisioners/shell: safety first

make it harder to forget -e when overriding shebang. Closes #2873

website/source/docs/provisioners/shell.html.md

@@ -220,9 +220,9 @@ would be:
 
 -   On Ubuntu, the `/bin/sh` shell is
     [dash](https://en.wikipedia.org/wiki/Debian_Almquist_shell). If your script
-    has [bash](https://en.wikipedia.org/wiki/Bash_(Unix_shell))-specific commands
-    in it, then put `#!/bin/bash` at the top of your script. Differences between
-    dash and bash can be found on the
+    has [bash](https://en.wikipedia.org/wiki/Bash_(Unix_shell))-specific
+    commands in it, then put `#!/bin/bash -e` at the top of your script.
+    Differences between dash and bash can be found on the
     [DashAsBinSh](https://wiki.ubuntu.com/DashAsBinSh) Ubuntu wiki page.
 
 *My shell works when I login but fails with the shell provisioner*