Nextcloud 13 is not compatible with newer than php 7.2

Just to avoid users from trying this with a to new (untested) php version

* Moved the check logic to 1 place
* All directly callable scripts just require this on top
* exit hard (-1) so we know scripts won't continue
* Return status 500 so no sync clients will try fancy stuff

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>