fix: emit output when bailing out

Turns out that shellcheck was right, and my bash-fu was weak. This
now properly emits output to stderr instead of a file named `2`.

Also it now uses `printf` instead of `echo` so that things like `\n` work.

scripts/merge-pr.sh

@@ -14,9 +14,7 @@ set -e -o pipefail
 function bail() {
     msg="$1"
     if [ -n "$msg" ]; then
-        # we do want to emit to stderr here
-        #shellcheck disable=SC2210
-        echo >2 "$msg"
+        >&2 printf "%s\n" "$msg"
     fi
     exit 1
 }