goprotobuf: update to new errors.
Mostly gofix, but a few needed hand-work,
such as in the generator.

R=r, bradfitz, rsc
CC=golang-dev
http://codereview.appspot.com/5335045
diff --git a/proto/equal.go b/proto/equal.go
index 405d99a..be235a6 100644
--- a/proto/equal.go
+++ b/proto/equal.go
@@ -37,7 +37,6 @@
 import (
 	"bytes"
 	"log"
-	"os"
 	"reflect"
 	"strings"
 )
@@ -190,7 +189,7 @@
 			log.Printf("proto: don't know how to compare extension %d of %v", extNum, base)
 			continue
 		}
-		var err os.Error
+		var err error
 		if m1 == nil {
 			m1, err = decodeExtension(e1.enc, desc)
 		}