device: return generic error from Ipc{Get,Set}Operation.

This makes uapi.go's public API conform to Go style in terms
of error types.

Signed-off-by: David Anderson <danderson@tailscale.com>

device/uapi.go

@@ -7,6 +7,7 @@ package device
 
 import (
 	"bufio"
+	"errors"
 	"fmt"
 	"io"
 	"net"
@@ -31,7 +32,7 @@ func (s IPCError) ErrorCode() int64 {
 	return s.int64
 }
 
-func (device *Device) IpcGetOperation(socket *bufio.Writer) *IPCError {
+func (device *Device) IpcGetOperation(socket *bufio.Writer) error {
 	lines := make([]string, 0, 100)
 	send := func(line string) {
 		lines = append(lines, line)
@@ -106,7 +107,7 @@ func (device *Device) IpcGetOperation(socket *bufio.Writer) *IPCError {
 	return nil
 }
 
-func (device *Device) IpcSetOperation(socket *bufio.Reader) *IPCError {
+func (device *Device) IpcSetOperation(socket *bufio.Reader) error {
 	scanner := bufio.NewScanner(socket)
 	logError := device.log.Error
 	logDebug := device.log.Debug
@@ -421,10 +422,20 @@ func (device *Device) IpcHandle(socket net.Conn) {
 
 	switch op {
 	case "set=1\n":
-		status = device.IpcSetOperation(buffered.Reader)
+		err = device.IpcSetOperation(buffered.Reader)
+		if !errors.As(err, &status) {
+			// should never happen
+			device.log.Error.Println("Invalid UAPI error:", err)
+			status = &IPCError{1}
+		}
 
 	case "get=1\n":
-		status = device.IpcGetOperation(buffered.Writer)
+		err = device.IpcGetOperation(buffered.Writer)
+		if !errors.As(err, &status) {
+			// should never happen
+			device.log.Error.Println("Invalid UAPI error:", err)
+			status = &IPCError{1}
+		}
 
 	default:
 		device.log.Error.Println("Invalid UAPI operation:", op)