Allow wxAutomationObject::GetInstance() create new instance if needed.

Allow wxAutomationObject::GetInstance() create new instance if needed.

When getting an instance of an OLE automation object, it is often useful to
connect to the existing instance if any or start a new one otherwise. Make
GetInstance() behave like this by default while still allowing to use the
wxAutomationInstance_UseExistingOnly flag to reestablish the old behaviour.

Also improve the error reporting in wxAutomationObject.

See #12489.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66262 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775