Disallow pairing JSON scan in existing wallet

[antonio-fr]

Since adding a specific Dojo server on a initialized wallet is not possible, the QRscanner in an existing wallet (top icon) should not validate a valid paring JSON string.
When scanning a pairing JSON (from the Dojo admin), the app opens the Networking panel and adds a Dojo line. There's only an unrelated error message when click on Enable.
The scanner in the wallet should display an error immediately, eventually in the payment transaction panel, as it is wit an invalid QR code.
I recommend to filter the data read by the scanner, only quit automatically the scanner to perform some actions, on specific "known" patterns.