ipn/ipnlocal, control/controlclient: don't propagate all map errors to UI

Fixes regression from 81cabf48ec which made
all map errors be sent to the frontend UI.

Fixes #3230

Change-Id: I7f142c801c7d15e268a24ddf901c3e6348b6729c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -452,10 +452,13 @@ func (b *LocalBackend) setClientStatus(st controlclient.Status) {
 		// TODO(crawshaw): display in the UI.
 		if errors.Is(st.Err, io.EOF) {
 			b.logf("[v1] Received error: EOF")
-		} else {
-			b.logf("Received error: %v", st.Err)
-			e := st.Err.Error()
-			b.send(ipn.Notify{ErrMessage: &e})
+			return
+		}
+		b.logf("Received error: %v", st.Err)
+		var uerr controlclient.UserVisibleError
+		if errors.As(st.Err, &uerr) {
+			s := uerr.UserVisibleError()
+			b.send(ipn.Notify{ErrMessage: &s})
 		}
 		return
 	}