added h5 clean-up for failed loadProject() calls

The _loadProject function (at projectManager.py:381) raises an error, and the caller catches a general Exception (at ilastikShell.py:1053) and can therefore not rely on anything being cleaned up, even if he calls ```_closeCurrentProject()```.

Addition:
```Exception.message``` seems to be deprecated, replaced it with ```str(Exception)```.


Steps to reproduce:

  1. start new workflow (e.g. pixel classification)
  2. add an image 
  3. save and close ilastik
  4. move the image
  5. restart ilastik 
  6. try to open the project
  7. in the dialog box, click on cancel
  8. start new workflow
  9. try to save as the same file
  10. h5py throws an error because it cannot open the same file twice