inform user of environment dependency (#2910)

This should prevent surprises for anyone else manually invoking
./bin/format.sh

Author: efx

bin/format.sh

@@ -1,3 +1,9 @@
-#!/bin/bash
+#!/usr/bin/env bash
+
+if [ -z "$EXERCISM_PRETTIER_VERSION" ]; then
+	echo "This script requires the EXERCISM_PRETTIER_VERSION variable to work."
+	echo "Please see docs/maintainers/style-guide.md for guidance."
+	exit 1
+fi
 
 npx "prettier@$EXERCISM_PRETTIER_VERSION" --write "**/*.{js,jsx,ts,tsx,css,sass,scss,html,json,md,yml}"