waitForScan
An optional part of the linking process. Intended to be called after displaying the QR code, will return quickly after the user scans the QR code. finishLink must be called after waitForScan returns a non-error.
If this function is not used, the finishLink function will automatically block waiting for a scan (the same functionality that exists currently).
Throws
if signald sends an error response or the incoming message is invalid
if the request to the socket fails