control/controlclient,tailcfg: propagate registration errors to the frontend

Signed-off-by: Maisem Ali <maisem@tailscale.com>

tailcfg/tailcfg.go

@@ -679,6 +679,10 @@ type RegisterResponse struct {
 	NodeKeyExpired    bool   // if true, the NodeKey needs to be replaced
 	MachineAuthorized bool   // TODO(crawshaw): move to using MachineStatus
 	AuthURL           string // if set, authorization pending
+
+	// Error indiciates that authorization failed. If this is non-empty,
+	// other status fields should be ignored.
+	Error string
 }
 
 // EndpointType distinguishes different sources of MapRequest.Endpoint values.

tailcfg/tailcfg_clone.go

@@ -243,6 +243,7 @@ var _RegisterResponseCloneNeedsRegeneration = RegisterResponse(struct {
 	NodeKeyExpired    bool
 	MachineAuthorized bool
 	AuthURL           string
+	Error             string
 }{})
 
 // Clone makes a deep copy of DERPRegion.