Don't return Server Error (500) on duplicate device registration

[iangudger]

If you register the same serial number twice, give a descriptive error message and let the user fix the problem. If you don't it makes your app seem broken.

[iangudger]

I also got a Server Error (500) when I tried to delete a device twice. Fix that too.

[aadabi]

Again good point @ianlofs and @nsokolnikov please take a note of this.