added a user not found error for permission grants

dtp-webapp-cli.js

@@ -34,6 +34,10 @@ module.grantPermission = async (target, permission) => {
   const User = mongoose.model('User');
   try {
     const user = await User.findOne({ email: target }).select('+permissions +flags');
+    if (!user) {
+      throw new Error(`User not found (email: ${target})`);
+    }
+
     switch (permission) {
       case 'admin':
         user.flags.isAdmin = true;