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/exec/exec.go b/src/pkg/exec/exec.go
index ebb40a2fe8..7ddb98b508 100644
--- a/src/pkg/exec/exec.go
+++ b/src/pkg/exec/exec.go
@@ -140,8 +140,8 @@ Error:
 // other options cause Wait to return for other
 // process events; see package os for details.
 func (p *Cmd) Wait(options int) (*os.Waitmsg, os.Error) {
-	if p.Pid < 0 {
-		return nil, os.EINVAL;
+	if p.Pid <= 0 {
+		return nil, os.ErrorString("exec: invalid use of Cmd.Wait");
 	}
 	w, err := os.Wait(p.Pid, options);
 	if w != nil && (w.Exited() || w.Signaled()) {
@@ -154,7 +154,7 @@ func (p *Cmd) Wait(options int) (*os.Waitmsg, os.Error) {
 // if it hasn't already, and then closes the non-nil file descriptors
 // p.Stdin, p.Stdout, and p.Stderr.
 func (p *Cmd) Close() os.Error {
-	if p.Pid >= 0 {
+	if p.Pid > 0 {
 		// Loop on interrupt, but
 		// ignore other errors -- maybe
 		// caller has already waited for pid.