library: io: move ErrAgain to shared and errPipe value to platform-specific

ErrAgain is not platform specific however EPIPE equivalent is different on Windows.

Signed-off-by: Ophestra Umiker <cat@ophivana.moe>

io.go

@@ -7,7 +7,10 @@ import (
 	"net"
 	"os"
 	"strconv"
-	"syscall"
+)
+
+var (
+	ErrAgain = errors.New("operation not performed")
 )
 
 type Client struct {
@@ -62,7 +65,7 @@ func Raw[T any](d *Client, opcode uint32, payload any) (uint32, T, error) {
 // Raw writes a raw payload to Discord and returns the response opcode and payload
 func (d *Client) Raw(opcode uint32, payload any) (uint32, []byte, error) {
 	opcodeResp, payloadResp, err := d.raw(opcode, payload)
-	if errors.Is(err, syscall.EPIPE) {
+	if errors.Is(err, errPipe) {
 		// clean up as much as possible
 		_ = d.Close()
 		// advise retry

io_unix.go

@@ -1,15 +1,16 @@
+//go:build !windows
+
 package rpcfetch
 
 import (
 	"errors"
 	"io/fs"
 	"net"
+	"syscall"
 	"time"
 )
 
-var (
-	ErrAgain = errors.New("operation not performed")
-)
+var errPipe = syscall.EPIPE
 
 func (d *Client) dial() error {
 	if d.dialed {