The change_view associated with BuildAdmin class was throwing
an exception when trying to delete a FirmwareImage object,
when the file associated with that object had already been
deleted from the file system.

There were two tasks according to my understanding:
1. Prevent the website to break and catch the error:
        the return expression in change_view has been put
in a try/catch expression, with instructions/hints/warnings
for the user using message_user

2. If an image file has been deleted from the filesystem,
automatically remove the FirmwareImage assiciated with it:

        The 'get_queryset' method for FirmwareImageInline class
has been over-written to automatically remove FirmwareImage objects
associated with deleted files and the information has been
logged with logger.

Fixes openwisp#140

…sp#140

I couldn't reproduce this issue with Django's standard `models.FileField`
so I figured that the problem is with the way `private_storage` is being
used. Strangely, there's no problem when delete method is called on an
instance of the model. The error is always thrown when the delete method
is called on a "modelForm" for an instance, the image file for which has
been deleted.

So the issue really is with the `private_storage` module. To get around
the error though, what I have done is:
 - `try` to return change_view
 - `catch` the `FileNotFoundError` if it occurs
 - Get the `path` from the error itself
 - check if the specified directory exists, if not create one
 - create a temporary file
 - return `change_view` again (recursion)
 - This time the file is there to be deleted and the model instance is
   deleted as well

This seems like a "brute force" solution, but since the origin of the issue
is an external module, this is the only possible workaround (As we don't want
to automatically delete any model instances because it would be strange for the
user)

Fixes openwisp#140

…sp#140

Added a try/catch block in the change_view of `BuildAdmin` class.
If a `FileNotFoundError` is caught, the relavant file/directory will be
temperorily re-created just before recursively calling change_view again.

Fixes openwisp#140

…sp#140

Added a try/catch block in the return statement in `BuildAdmin` `change_view`

If a `FileNotFoundError` is caught, the file/directory from the error path
is recreated temperorily before recursively calling `change_view` again, to
complete the delete operation without throwing any errors.

Fixes openwisp#140