Change os.Error convention:

echo back context of call in error if likely to be useful.

For example, if os.Open("/etc/passwd", os.O_RDONLY)
fails with syscall.EPERM, it returns as the os.Error

	&PathError{
		Op: "open",
		Path: "/etc/passwd"
		Error: os.EPERM
	}

which formats as

	open /etc/passwd: permission denied

Not converted:

	datafmt
	go/...
	google/...
	regexp
	tabwriter
	template

R=r
DELTA=1153  (561 added, 156 deleted, 436 changed)
OCL=30738
CL=30781

src/pkg/net/dialgoogle_test.go

@@ -6,7 +6,6 @@ package net
 
 import (
 	"flag";
-	"fmt";
 	"io";
 	"net";
 	"os";