dpbx: doesn't work with par2 backend

As far as I understand, it should be possible to add/remove par2 wrapper at any time. For dpbx backend it doesn't work because par2 expects certain kind of exception for 'file not found' error:

dpbx,files_download(/test/duplicity-full-signatures.20160530T073719Z.sigtar.gz.par2)
Exception [('ac9210f954bd7235a5e59e88a05b9624', DownloadError(u'path', LookupError(u'not_found', None)))]:
| Traceback (most recent call last):
| File "/home/dion/dev/duplicity/duplicity/backends/dpbxbackend.py", line 76, in wrapper
| return f(self, *args)
| File "/home/dion/dev/duplicity/duplicity/backends/dpbxbackend.py", line 301, in _get
| res_metadata, http_fd = self.api_client.files_download(remote_path)
| File "/home/dion/dev/dropbox-sdk-python/dropbox/base.py", line 139, in files_download
| None,
| File "/home/dion/dev/dropbox-sdk-python/dropbox/dropbox.py", line 244, in request
| user_message_locale)
| ApiError: ('ac9210f954bd7235a5e59e88a05b9624', DownloadError(u'path', LookupError(u'not_found', None)))
dpbx code error "('ac9210f954bd7235a5e59e88a05b9624', DownloadError(u'path', LookupError(u'not_found', None)))"
Deleting tree /tmp/duplicity-Cv8NXL-tempdir/duplicity_temp.1
Selecting /tmp/duplicity-Cv8NXL-tempdir/duplicity_temp.1
Deleting /tmp/duplicity-Cv8NXL-tempdir/duplicity_temp.1
Backtrace of previous error: Traceback (innermost last):
  File "/home/dion/dev/duplicity/duplicity/backend.py", line 365, in inner_retry
    return fn(self, *args)
  File "/home/dion/dev/duplicity/duplicity/backend.py", line 541, in get
    self.backend._get(remote_filename, local_path)
  File "/home/dion/dev/duplicity/duplicity/backends/par2backend.py", line 114, in get
    self.wrapped_backend._get(par2file.get_filename(), par2file)
  File "/home/dion/dev/duplicity/duplicity/backends/dpbxbackend.py", line 76, in wrapper
    return f(self, *args)
  File "/home/dion/dev/duplicity/duplicity/backends/dpbxbackend.py", line 301, in _get
    res_metadata, http_fd = self.api_client.files_download(remote_path)
  File "/home/dion/dev/dropbox-sdk-python/dropbox/base.py", line 139, in files_download
    None,
  File "/home/dion/dev/dropbox-sdk-python/dropbox/dropbox.py", line 244, in request
    user_message_locale)
 ApiError: ('ac9210f954bd7235a5e59e88a05b9624', DownloadError(u'path', LookupError(u'not_found', None)))

I'm not sure, whether par2 expectation is correct or not. In any case followed patch fixes par2+dpbx backend so it's now possible to add/remove par2 at any time