internal/encoding/wire: switch to use internal errors package

Change-Id: Ibb01f8a5d4dea09c19262f807e305bbebe0e6ab8
Reviewed-on: https://go-review.googlesource.com/127825
Reviewed-by: Herbie Ong <herbie@google.com>

internal/encoding/wire/wire.go

@@ -8,9 +8,10 @@
 package wire
 
 import (
-	"errors" // TODO: replace with our own errors package?
 	"io"
 	"math/bits"
+
+	"google.golang.org/proto/internal/errors"
 )
 
 // Number represents the field number.