exec.LookPath: return os.PathError instad of os.ENOENT, it's more descriptive.

R=rsc
CC=golang-dev
https://golang.org/cl/3448042

src/pkg/exec/lp_unix.go

@@ -29,7 +29,7 @@ func LookPath(file string) (string, os.Error) {
 		if canExec(file) {
 			return file, nil
 		}
-		return "", os.ENOENT
+		return "", &os.PathError{"lookpath", file, os.ENOENT}
 	}
 	pathenv := os.Getenv("PATH")
 	for _, dir := range strings.Split(pathenv, ":", -1) {
@@ -41,5 +41,5 @@ func LookPath(file string) (string, os.Error) {
 			return dir + "/" + file, nil
 		}
 	}
-	return "", os.ENOENT
+	return "", &os.PathError{"lookpath", file, os.ENOENT}
 }

src/pkg/exec/lp_windows.go

@@ -49,7 +49,7 @@ func LookPath(file string) (string, os.Error) {
 		if f, ok := canExec(file, exts); ok {
 			return f, nil
 		}
-		return ``, os.ENOENT
+		return ``, &os.PathError{"lookpath", file, os.ENOENT}
 	}
 	if pathenv := os.Getenv(`PATH`); pathenv == `` {
 		if f, ok := canExec(`.\`+file, exts); ok {
@@ -62,5 +62,5 @@ func LookPath(file string) (string, os.Error) {
 			}
 		}
 	}
-	return ``, os.ENOENT
+	return ``, &os.PathError{"lookpath", file, os.ENOENT}
 }