Import the explicit exception

Since we aren't importing the sh module wholly, we need to hand-import
all things we use from it.

Signed-off-by: Ritesh Raj Sarraf <ritesh.sarraf@collabora.com>

package-source-builder/overlay/usr/bin/apertis-pkg-merge-upstream-to-downstreams

@@ -11,6 +11,7 @@ import argparse
 from sh.contrib import git
 from sh import apertis_pkg_merge_updates
 from sh import echo
+from sh import ErrorReturnCode
 import re
 from pathlib import Path
 from functools import partial