editor: fix error handling when requesting secrets from non-existent setting

When NM core switched over to libnm-core and errors were consolidated,
the error name changed.  Recognize the new one.  Fixed an issue where
editing any WWAN connection would fail because secrets for the PPP
setting were requested (for GSM username/password) but connections
aren't actually required to have a PPP setting if no user/password
is required.