Fix wrong logic for CFOP and nfe

The CFOP field should be enabled when the nfe plugin is active.

Change-Id: I3e2aa47499b796470dad16d70f84daa0b28d0e56
Fixes: http://bugs.async.com.br/show_bug.cgi?id=5678
Fix a couple of errors:

- Mark a string for translation
- Convert an exception into an string before trying to show it to a user.

Fixes CR 2797

Change-Id: Idfb0f933116a8fb884596715ebc61c2e8d357f3d