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

1 files changed, 3 insertions, 3 deletions

diff --git a/src/pkg/os/dir_linux_amd64.go b/src/pkg/os/dir_linux_amd64.go
index 05b3d4c65..8b1664f77 100644
--- a/src/pkg/os/dir_linux_amd64.go
+++ b/src/pkg/os/dir_linux_amd64.go
@@ -43,10 +43,10 @@ func readdirnames(file *File, count int) (names []string, err Error) {
 		if d.bufp >= d.nbuf {
 			var errno int;
 			d.nbuf, errno = syscall.Getdents(file.fd, d.buf);
-			if d.nbuf < 0 {
-				return names, ErrnoToError(errno)
+			if errno != 0 {
+				return names, NewSyscallError("getdents", errno)
 			}
-			if d.nbuf == 0 {
+			if d.nbuf <= 0 {
 				break	// EOF
 			}
 			d.bufp = 0;