chg: dev: [vps] fail early on unexpected errors due to misconfigured ``compose.yml``

Signed-off-by: Valentin Lab <valentin.lab@kalysto.org>

bin/vps

@@ -327,8 +327,8 @@ compose:install-backup() {
                 "entry in '$compose_file'."
             return 1
         }
-        private_key=$(e "$cfg" | shyaml get-value private-key)
-        target=$(e "$cfg" | shyaml get-value target)
+        private_key=$(e "$cfg" | shyaml get-value private-key) || return 1
+        target=$(e "$cfg" | shyaml get-value target) || return 1
         if [ "$target" != "$BACKUP_SERVER" ]; then
             err "Existing backup target '$target' is different" \
                 "from specified '$BACKUP_SERVER'"