fdo::DBusProxy methods should use fdo::Error

Update: This issue was originally created for the discussion on !111 (merged) linked below but after some thinking and discussions here, the scope changed to what the title now says.

The following discussion from !111 (merged) should be addressed:

• @zeenix started a discussion: (+14 comments)

  Where did you get the list of these errors? I only see a portion of them mentioned on the D-Bus spec.