gob: have errorf always prefix the message with "gob: "
to regularize the errors.

R=rsc
CC=golang-dev
https://golang.org/cl/4446055
diff --git a/src/pkg/gob/debug.go b/src/pkg/gob/debug.go
index 69c83bd..79aee77 100644
--- a/src/pkg/gob/debug.go
+++ b/src/pkg/gob/debug.go
@@ -335,7 +335,7 @@
 func (deb *debugger) delta(expect int) int {
 	delta := int(deb.uint64())
 	if delta < 0 || (expect >= 0 && delta != expect) {
-		errorf("gob decode: corrupted type: delta %d expected %d", delta, expect)
+		errorf("decode: corrupted type: delta %d expected %d", delta, expect)
 	}
 	return delta
 }