Show friendly 500 error page to users and developers (#24110)

Close #24104

This also introduces many tests to cover many complex error handling
functions.

### Before

The details are never shown in production.

<details>

![image](https://user-images.githubusercontent.com/2114189/231805004-13214579-4fbe-465a-821c-be75c2749097.png)

</details>

### After

The details could be shown to site admin users. It is safe.

![image](https://user-images.githubusercontent.com/2114189/231803912-d5660994-416f-4b27-a4f1-a4cc962091d4.png)

modules/templates/htmlrenderer_test.go

@@ -0,0 +1,106 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package templates
+
+import (
+	"errors"
+	"html/template"
+	"os"
+	"strings"
+	"testing"
+
+	"code.gitea.io/gitea/modules/assetfs"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestExtractErrorLine(t *testing.T) {
+	cases := []struct {
+		code   string
+		line   int
+		pos    int
+		target string
+		expect string
+	}{
+		{"hello world\nfoo bar foo bar\ntest", 2, -1, "bar", `
+foo bar foo bar
+    ^^^     ^^^
+`},
+
+		{"hello world\nfoo bar foo bar\ntest", 2, 4, "bar", `
+foo bar foo bar
+    ^
+`},
+
+		{
+			"hello world\nfoo bar foo bar\ntest", 2, 4, "",
+			`
+foo bar foo bar
+    ^
+`,
+		},
+
+		{
+			"hello world\nfoo bar foo bar\ntest", 5, 0, "",
+			`unable to find target line 5`,
+		},
+	}
+
+	for _, c := range cases {
+		actual := extractErrorLine([]byte(c.code), c.line, c.pos, c.target)
+		assert.Equal(t, strings.TrimSpace(c.expect), strings.TrimSpace(actual))
+	}
+}
+
+func TestHandleError(t *testing.T) {
+	dir := t.TempDir()
+
+	p := &templateErrorPrettier{assets: assetfs.Layered(assetfs.Local("tmp", dir))}
+
+	test := func(s string, h func(error) string, expect string) {
+		err := os.WriteFile(dir+"/test.tmpl", []byte(s), 0o644)
+		assert.NoError(t, err)
+		tmpl := template.New("test")
+		_, err = tmpl.Parse(s)
+		assert.Error(t, err)
+		msg := h(err)
+		assert.EqualValues(t, strings.TrimSpace(expect), strings.TrimSpace(msg))
+	}
+
+	test("{{", p.handleGenericTemplateError, `
+template error: tmp:test:1 : unclosed action
+----------------------------------------------------------------------
+{{
+----------------------------------------------------------------------
+`)
+
+	test("{{Func}}", p.handleFuncNotDefinedError, `
+template error: tmp:test:1 : function "Func" not defined
+----------------------------------------------------------------------
+{{Func}}
+  ^^^^
+----------------------------------------------------------------------
+`)
+
+	test("{{'x'3}}", p.handleUnexpectedOperandError, `
+template error: tmp:test:1 : unexpected "3" in operand
+----------------------------------------------------------------------
+{{'x'3}}
+     ^
+----------------------------------------------------------------------
+`)
+
+	// no idea about how to trigger such strange error, so mock an error to test it
+	err := os.WriteFile(dir+"/test.tmpl", []byte("god knows XXX"), 0o644)
+	assert.NoError(t, err)
+	expectedMsg := `
+template error: tmp:test:1 : expected end; found XXX
+----------------------------------------------------------------------
+god knows XXX
+          ^^^
+----------------------------------------------------------------------
+`
+	actualMsg := p.handleExpectedEndError(errors.New("template: test:1: expected end; found XXX"))
+	assert.EqualValues(t, strings.TrimSpace(expectedMsg), strings.TrimSpace(actualMsg))
+}