handle gracefully -f (--find-links) in "pip freeze" and report error in ...

...output

this bug had been introduced in 0.16

pip freeze can return more then ``-e`` and ``==``.

the if/elif logic caused this:

```
   File "/usr/lib/pymodules/python2.6/salt/modules/pip.py", line 574, in list_
     packages[name] = version
UnboundLocalError: local variable 'version' referenced before assignment
```

when pip freeze output isn't handled, it log it as an error. but I think this should make pip.list fail.
what is the best to do in salt modules that don't return a dict ``{'result': False, ...}`` ?