Bug 14070: Prevent to save a letter if an error occurred

Bug 14070: Prevent to save a letter if an error occurred

On saving a letter, if the title or the content of a template is not
defined, a JS alert is raised. But the form is submitted anyway.
This patch prevent the form to be submitted.

Test plan:
- Create or edit a letter
- Fill the title for a template, not the content
- Save
- Confirm you get the alert and that the form is not submitted

http://bugs.koha-community.org/show_bug.cgi?id=14070

Signed-off-by: Indranil Das Gupta <indradg@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>