[admin] Fixed FileNotFoundError while deleting a FirmwareImage openwi…

…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