Reads the installID stored in a cookie by the WriteInstallID API.
Use the FavorMachineLevelLicenseKey to control which key takes precedence if the
keys are stored at the user and machine level.
This function requires that the QlmAnalytics.GUID property be initialized to the GUID associated to your product.
C#: string ReadInstallID(out string errorMessage)
- errorMessage - the error message if the operation failed.
Returns the install ID.