maintainers/release-notes: Let it fail

Fail when a command fails.

Basic error handling was missing, which would lead to errors getting
obscured a bit by subsequent successful logging.

maintainers/release-notes

@@ -2,6 +2,8 @@
 # vim: set filetype=bash:
 #!nix shell .#changelog-d --command bash
 
+set -euo pipefail
+
 # --- CONFIGURATION ---
 
 # This does double duty for