Use-before-defined in exception handler for App constructor

[ChrisTimperley]

  File "<attrs generated init roswire.app.app.App>", line 7, in __init__
  File "/home/chris/tools/roswire/src/roswire/app/app.py", line 52, in __attrs_post_init__
    raise exc.ImageNotFound(image) from err
UnboundLocalError: local variable 'image' referenced before assignment