Output changelog error to stderr

Author: camsteffen

.github/workflows/clippy_bors.yml

@@ -39,7 +39,7 @@ jobs:
           grep "^changelog: " | \
           sed "s/changelog: //g")
         if [[ -z "$output" ]]; then
-          echo "ERROR: PR body must contain 'changelog: ...'"
+          echo "ERROR: PR body must contain 'changelog: ...'" >&2
           exit 1
         elif [[ "$output" = "none" ]]; then
           echo "WARNING: changelog is 'none'"