1. Catch NameError on import (could be raised by case mismatch on Windows).

1. Catch NameError on import (could be raised by case mismatch on Windows).
2. No longer need to reset pyclbr cache and show watch cursor when calling
   ClassBrowser -- the ClassBrowser takes care of pyclbr and the TreeWidget
   takes care of the watch cursor.
3. Reset the focus to the current window after error message about class
   browser on buffer without filename.