Add exit flag to postinst

[loleg]

On my linux server (Ubuntu 22.04.2 LTS) with Mattermost Omnibus installed, I had the issue of being unable to upgrade or downgrade Mattermost. I kept getting errors like this:

Setting up mattermost (7.10.0-0) ...
dpkg: error processing package mattermost (--configure):
 installed mattermost package post-installation script subprocess returned error exit status 1
Errors were encountered while processing:
 mattermost

I also tried a complete uninstall and reinstall. Debug mode gave me a clue to an issue with the installation script:

sudo dpkg --configure --debug=77777 mattermost

After adding an exit 0 to the end of the install script, I could finally finish the package configuration.