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

src/pkg/net/net_test.go

@@ -0,0 +1,72 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package net
+
+import (
+	"net";
+	"os";
+	"regexp";
+	"testing";
+)
+
+type DialErrorTest struct {
+	Net string;
+	Laddr string;
+	Raddr string;
+	Pattern string;
+}
+
+var dialErrorTests = []DialErrorTest {
+	DialErrorTest{
+		"datakit", "", "mh/astro/r70",
+		"dial datakit mh/astro/r70: unknown network datakit",
+	},
+	DialErrorTest{
+		"tcp", "", "127.0.0.1:☺",
+		"dial tcp 127.0.0.1:☺: unknown port tcp/☺",
+	},
+	DialErrorTest{
+		"tcp", "", "no-such-name.google.com.:80",
+		"dial tcp no-such-name.google.com.:80: lookup no-such-name.google.com.: no such host",
+	},
+	DialErrorTest{
+		"tcp", "", "no-such-name.no-such-top-level-domain.:80",
+		"dial tcp no-such-name.no-such-top-level-domain.:80: lookup no-such-name.no-such-top-level-domain.: no such host",
+	},
+	DialErrorTest{
+		"tcp", "", "no-such-name:80",
+		"dial tcp no-such-name:80: lookup no-such-name.google.com.: no such host",
+	},
+	DialErrorTest{
+		"tcp", "", "mh/astro/r70:http",
+		"dial tcp mh/astro/r70:http: lookup mh/astro/r70: invalid domain name",
+	},
+	DialErrorTest{
+		"unix", "", "/etc/file-not-found",
+		"dial unix /etc/file-not-found: no such file or directory",
+	},
+	DialErrorTest{
+		"unix", "", "/etc/",
+		"dial unix /etc/: (permission denied|socket operation on non-socket)",
+	},
+}
+
+func TestDialError(t *testing.T) {
+	for i, tt := range dialErrorTests {
+		c, e := net.Dial(tt.Net, tt.Laddr, tt.Raddr);
+		if c != nil {
+			c.Close();
+		}
+		if e == nil {
+			t.Errorf("#%d: nil error, want match for %#q", i, tt.Pattern);
+			continue;
+		}
+		s := e.String();
+		match, err := regexp.Match(tt.Pattern, s);
+		if !match {
+			t.Errorf("#%d: %q, want match for %#q", i, s, tt.Pattern);
+		}
+	}
+}