The two errors that you describe have to do with attempting to install a non-standard iOS. You say that you tried to install iOS 8.1.2. That cannot be done, even attempting it with DFU Mode. You can only restore to the latest iOS available for your device which is iOS 8.4.1. As far as the error 53, it could be a hardware error or a security software error. Take a look at this support document. Get help with iOS update and restore errors - Apple Support
Errors: 1, 3, 11, 12, 13, 14, 16, 20, 21, 23, 26, 27, 28, 29, 34, 35, 36, 37, 40, 53, 56, 1002, 1004, 1011, 1012, 1014, 1667, or 1669.
Your device or computer might have a hardware issue that stops the update or restore from completing.
- Check that your security software and settings aren't blocking your device from communicating with the Apple update server.
- Then try to restore your iOS device two more times while connected with a cable, computer, and network you know are good.
- Make sure that your security software and settings are allowing communication between your device and update servers.
- If you still see the error message when you update or restore, contact Apple support.
If you still see the error message, check for outdated or modified software.
You may have made enough attempts to modify your device that you have caused the problems. You might try making an appointment at the Genius Bar of the nearest Apple Store to get assistance unless you have tried to jailbreak the device and in that case they will not look at the device.