Return valid code MTRError when commissioning is attempted with duplicate NOC

[skothawade-apple]

Problem

MTRError doesn't define code for duplicate fabric, therefore clients receive MTRError with default code 1. Here is how error message looks like-

Error Domain=MTRErrorDomain Code=1 "Undefined error:126." UserInfo={underlyingError=<MTRErrorHolder: 0x106223f20>, NSLocalizedDescription=Undefined error:126., errorCode=126}

Proposed Solution

We should define a new MTRError code and return that instead of above

[aajain-com]

This is needed so we can gracefully handle cases where the accessory is already added to the fabric previously.

[bzbarsky-apple]

Note to self: that's CHIP_ERROR_FABRIC_EXISTS.