commit a0bcaf4c00163e90133981dff2e1524f2133b170
author Russ Cox <rsc@golang.org> Thu Jun 25 20:24:55 2009 -0700
committer Russ Cox <rsc@golang.org> Thu Jun 25 20:24:55 2009 -0700
tree 1596fd2f89c1d896cdf5772aebc910f4e0ff5bda
parent 70e232e668e459c9f730a2213f8270660b36558b

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