Synchronize setting of QBluetoothSocket's error string, code and signal

This fixes a few cases where errorString, error signal and error code
where out of sync because one was set but not the other. This was
addressed by unifying the usage pattern for error activation to a
pattern where errorString is set and setSocketError() is called
afterwards.

Change-Id: Ibfb04772cf560936aa4ce8ea9643d6a410cc9ee2
Reviewed-by: Fabian Bumberger <fbumberger@rim.com>
Use qt_error_string rather than strerror.

Change-Id: Ide3f00cacd7a42e99823441a45609f8e50063fce
Reviewed-by: Fabian Bumberger <fbumberger@rim.com>