html/template, text/template: drop defined template list from errors

The report in #17414 points out that if you have many many templates,
then this is an overwhelming list and just hurts the signal-to-noise ratio of the error.

Even the test of the old behavior also supports the idea that this is noise:

	template: empty: "empty" is an incomplete or empty template; defined templates are: "secondary"

The chance that someone mistyped "secondary" as "empty" is slim at best.

Similarly, the compiler does not augment an error like 'unknown variable x'
by dumping the full list of all the known variables.

For all these reasons, drop the list.

Fixes #17414.

Change-Id: I78f92d2c591df7218385fe723a4abc497913acf8
Reviewed-on: https://go-review.googlesource.com/32116
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/html/template/template.go

@@ -96,7 +96,7 @@ func (t *Template) escape() error {
 	t.nameSpace.escaped = true
 	if t.escapeErr == nil {
 		if t.Tree == nil {
-			return fmt.Errorf("template: %q is an incomplete or empty template%s", t.Name(), t.DefinedTemplates())
+			return fmt.Errorf("template: %q is an incomplete or empty template", t.Name())
 		}
 		if err := escapeTemplate(t, t.text.Root, t.Name()); err != nil {
 			return err

src/text/template/exec.go

@@ -190,7 +190,7 @@ func (t *Template) execute(wr io.Writer, data interface{}) (err error) {
 		vars: []variable{{"$", value}},
 	}
 	if t.Tree == nil || t.Root == nil {
-		state.errorf("%q is an incomplete or empty template%s", t.Name(), t.DefinedTemplates())
+		state.errorf("%q is an incomplete or empty template", t.Name())
 	}
 	state.walk(value, t.Root)
 	return

src/text/template/exec_test.go

@@ -932,7 +932,7 @@ func TestMessageForExecuteEmpty(t *testing.T) {
 		t.Fatal("expected second error")
 	}
 	got = err.Error()
-	want = `template: empty: "empty" is an incomplete or empty template; defined templates are: "secondary"`
+	want = `template: empty: "empty" is an incomplete or empty template`
 	if got != want {
 		t.Errorf("expected error %s got %s", want, got)
 	}