(Classic) Bluetooth discovery agents to clear previous errors

Bluetooth device- or service discovery may result in an error for a number of reasons.

When the discovery is retried, the previous errors should be cleared. As an example if bluetooth was OFF the discovery will result in a PoweredOff error. Switching the bluetooth ON and redoing a successful discovery should clear the previous error. Otherwise the discoveryAgent::error() will return "PoweredOff" error even though everything was successful.

This task is about fixing this on the applicable platforms (analysis of the impacted platforms is part of this item).