Use --if-exists with import

To resolve https://github.com/dokku/dokku-postgres/issues/142

functions

@@ -114,7 +114,7 @@ service_import() {
   if [[ -t 0 ]]; then
     dokku_log_fail "No data provided on stdin."
   fi
-  docker exec -i "$SERVICE_NAME" env PGPASSWORD="$PASSWORD" pg_restore -h localhost -cO -d "$DATABASE_NAME" -U postgres -w
+  docker exec -i "$SERVICE_NAME" env PGPASSWORD="$PASSWORD" pg_restore -h localhost -cO --if-exists -d "$DATABASE_NAME" -U postgres -w
 }
 
 service_start() {