net: move error messages related to OpError into net.go

Also makes ErrWriteToConnected more appropriate; it's used
not only UDPConn operations but UnixConn operations.

Update #4856

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/84800044

src/pkg/net/net.go

@@ -275,7 +275,16 @@ type Listener interface {
 	Addr() Addr
 }
 
-var errMissingAddress = errors.New("missing address")
+// Various errors contained in OpError.
+var (
+	// For connection setup and write operations.
+	errMissingAddress = errors.New("missing address")
+
+	// For both read and write operations.
+	errTimeout          error = &timeoutError{}
+	errClosing                = errors.New("use of closed network connection")
+	ErrWriteToConnected       = errors.New("use of WriteTo with pre-connected connection")
+)
 
 // OpError is the error type usually returned by functions in the net
 // package. It describes the operation, network type, and address of
@@ -337,10 +346,6 @@ func (e *timeoutError) Error() string   { return "i/o timeout" }
 func (e *timeoutError) Timeout() bool   { return true }
 func (e *timeoutError) Temporary() bool { return true }
 
-var errTimeout error = &timeoutError{}
-
-var errClosing = errors.New("use of closed network connection")
-
 type AddrError struct {
 	Err  string
 	Addr string

src/pkg/net/udpsock.go

@@ -4,10 +4,6 @@
 
 package net
 
-import "errors"
-
-var ErrWriteToConnected = errors.New("use of WriteTo with pre-connected UDP")
-
 // UDPAddr represents the address of a UDP end point.
 type UDPAddr struct {
 	IP   IP