Restore correct handling of wxDocument::OnCreate() error return value.

The changes of r74515 didn't quite restore the old behaviour, the document was
still not being cleaned up if its OnCreate() simply returned false and not
threw an exception.

Do add cleanup in this code branch too, just duplicating what we in case of
exception (this duplication can't be easily avoided unfortunately).

This backports changes of r75646 from trunk, see #15883.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76315 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775