TransformToolUI : Fix bug in status tooltip generation
TransformToolUI : Fix bug in status tooltip generation

A non-editable selection would lead to the following stacktrace :

```
Traceback (most recent call last):
  File "/disk1/john/dev/build/gaffer/python/GafferUI/Widget.py", line 933, in eventFilter
    return self.__toolTip( qObject, qEvent )
  File "/disk1/john/dev/build/gaffer/python/GafferUI/Widget.py", line 992, in __toolTip
    toolTip = widget.getToolTip()
  File "/disk1/john/dev/build/gaffer/python/GafferSceneUI/TransformToolUI.py", line 146, in getToolTip
    script = toolSelection[0].editTarget().ancestor( Gaffer.ScriptNode )
IECore.Exception: Selection is not editable
```

We don't need to deal with non-editable selections in `getToolTip()` because we've already put a more specific tooltip on the warning indicator in this case (in `__update()`).

johnhaddon/gaffer,boberfly/gaffer,hradec/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,hradec/gaffer,GafferHQ/gaffer,hradec/gaffer,lucienfostier/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,boberfly/gaffer,hradec/gaffer,boberfly/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer,GafferHQ/gaffer,hradec/gaffer,ImageEngine/gaffer,johnhaddon/gaffer,boberfly/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer