Fix lintian's maintainer-script-ignores-errors

* Error checking of every command was enabled according to documentation
  (Debian Policy Manual, Section 10.6. Scripts):
  https://www.debian.org/doc/debian-policy/ch-files.html#scripts

packaging/debian/postinst

@@ -6,6 +6,9 @@
 # Use of this source is governed by the MIT license, see LICENSE file.
 #-------------------------------------------------------------------------------
 
+# Check exit status of any command and fail on error
+set -e
+
 # Source global configurations
 . /etc/default/mentat