Warn user if missing a config file

Report a warning when the main json file is missing and there is a
local or {env} file present

lib/config-loader.js

@@ -89,6 +89,10 @@ function loadNamed(rootDir, env, name, mergeFn) {
  */
 function findConfigFiles(appRootDir, env, name) {
   var master = ifExists(name + '.json');
+  if (!master && (ifExistsWithAnyExt(name + '.local') ||
+    ifExistsWithAnyExt(name + '.' + env))) {
+    console.warn('WARNING: Main config file "' + name + '.json" is missing');
+  }
   if (!master) return [];
 
   var candidates = [