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>

control/controlclient/client.go

@@ -79,3 +79,9 @@ type Client interface {
 	// requesting a DNS record be created or updated.
 	SetDNS(context.Context, *tailcfg.SetDNSRequest) error
 }
+
+// UserVisibleError is an error that should be shown to users.
+type UserVisibleError string
+
+func (e UserVisibleError) Error() string            { return string(e) }
+func (e UserVisibleError) UserVisibleError() string { return string(e) }

control/controlclient/direct.go

@@ -431,7 +431,7 @@ func (c *Direct) doLogin(ctx context.Context, opt loginOpt) (mustRegen bool, new
 		resp.NodeKeyExpired, resp.MachineAuthorized, resp.AuthURL != "")
 
 	if resp.Error != "" {
-		return false, "", errors.New(resp.Error)
+		return false, "", UserVisibleError(resp.Error)
 	}
 	if resp.NodeKeyExpired {
 		if regen {

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
 	}