device: introduce new IPC error message for unknown error

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

device/uapi.go

@@ -427,8 +427,8 @@ func (device *Device) IpcHandle(socket net.Conn) {
 	// write status
 	var status *IPCError
 	if err != nil && !errors.As(err, &status) {
-		// I/O error, maybe something unexpected
-		status = ipcErrorf(1, "other UAPI error: %w", err)
+		// shouldn't happen
+		status = ipcErrorf(ipc.IpcErrorUnknown, "other UAPI error: %w", err)
 	}
 	if status != nil {
 		device.log.Error.Println(status)

ipc/uapi_unix.go

@@ -21,6 +21,7 @@ const (
 	IpcErrorProtocol  = -int64(unix.EPROTO)
 	IpcErrorInvalid   = -int64(unix.EINVAL)
 	IpcErrorPortInUse = -int64(unix.EADDRINUSE)
+	IpcErrorUnknown   = -55 // ENOANO
 )
 
 // socketDirectory is variable because it is modified by a linker