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, 16 insertions, 4 deletions

diff --git a/doc/progs/file.go b/doc/progs/file.go
index ff796c565..bda3890de 100644
--- a/doc/progs/file.go
+++ b/doc/progs/file.go
@@ -29,7 +29,10 @@ var (
 
 func Open(name string, mode int, perm int) (file *File, err os.Error) {
 	r, e := syscall.Open(name, mode, perm);
-	return newFile(r, name), os.ErrnoToError(e)
+	if e != 0 {
+		err = os.Errno(e);
+	}
+	return newFile(r, name), err
 }
 
 func (file *File) Close() os.Error {
@@ -38,7 +41,10 @@ func (file *File) Close() os.Error {
 	}
 	e := syscall.Close(file.fd);
 	file.fd = -1;  // so it can't be closed again
-	return os.ErrnoToError(e);
+	if e != 0 {
+		return os.Errno(e);
+	}
+	return nil
 }
 
 func (file *File) Read(b []byte) (ret int, err os.Error) {
@@ -46,7 +52,10 @@ func (file *File) Read(b []byte) (ret int, err os.Error) {
 		return -1, os.EINVAL
 	}
 	r, e := syscall.Read(file.fd, b);
-	return int(r), os.ErrnoToError(e)
+	if e != 0 {
+		err = os.Errno(e);
+	}
+	return int(r), err
 }
 
 func (file *File) Write(b []byte) (ret int, err os.Error) {
@@ -54,7 +63,10 @@ func (file *File) Write(b []byte) (ret int, err os.Error) {
 		return -1, os.EINVAL
 	}
 	r, e := syscall.Write(file.fd, b);
-	return int(r), os.ErrnoToError(e)
+	if e != 0 {
+		err = os.Errno(e);
+	}
+	return int(r), err
 }
 
 func (file *File) String() string {