Do not set error messages when importing fields that are not recognized

As uif_plus is not necessarily the only module adding handlers for the import it might be a bit too much to say fields are ignored when importing.

I have another contrib which handles some fields not handled by the uif_plus (addressfield) yet and I get messages about fields beeing ignored, while they are actually beeing processed.
It might be better to not show the message at all or reverse it (status message that a field is recognized by uif_plus).

Any thoughts?