duplicity prints a non-fatal/misleading error message

[Cristian Ciupitu]

This is basically a clone of https://bugzilla.redhat.com/show_bug.cgi?id=1048656

Description of problem:
duplicity prints a message which looks like a fatal error, but it isn't.
It complains about the dropbox backend missing, even if it's not used.
In my opinion an error message should only be printed even needed.

A possible solution would be to make that module mandatory, but it would
be nicer if duplicity would be more modular.

Version-Release number of selected component (if applicable):
duplicity-0.6.22-1.fc20.x86_64 (Fedora 20 x86_64)
python-2.7.5-9.fc20.x86_64

How reproducible:
At least once, but it probably happens every time.

Steps to Reproduce:
1. duplicity restore --file-to-restore xxx file:///yyy/ /tmp/zzz/ # ext4 or XFS filesystem

Actual results:
Import of duplicity.backends.dpbxbackend Failed: No module named dropbox

Expected results:
No message. No news is good news.

Additional info:
I don't think it matters, but the backup was created under Fedora 19.