fix: show error when frontend HTML entry does not exist (#2289)

* fix: add response when web file not found

The error flow is as follows:

Assuming my directory structure is as follows:

```tree
├── GitHub
│   ├── casdoor  # code repository
              ├── casdoor # compiled binary file
```

Execute the program in the `GitHub` directory:

```bash
./casdoor/casdoor
```

The working directory at this time is `GitHub`.

According to the code:

```go
func StaticFilter(ctx *context.Context) {
	urlPath := ctx.Request.URL.Path

   /// omitted

	path := "web/build"
	if urlPath == "/" {
		path += "/index.html"
	} else {
		path += urlPath
	}

	if !util.FileExist(path) {
		path = "web/build/index.html"
	}
	if !util.FileExist(path) {
		return
	}

    /// omitted
}
```

If the user accesses `/`, according to this code, the returned value is actually `web/build/index.html`. But the current directory is GitHub, and there is no `web/build/index.html` file. According to the following code, it will directly return:

```go
	if !util.FileExist(path) {
		return
	}
```

Then in `main.go`:

```go
	beego.InsertFilter("*", beego.BeforeRouter, routers.StaticFilter)
	beego.InsertFilter("*", beego.BeforeRouter, routers.AutoSigninFilter)
	beego.InsertFilter("*", beego.BeforeRouter, routers.CorsFilter)
	beego.InsertFilter("*", beego.BeforeRouter, routers.ApiFilter)
	beego.InsertFilter("*", beego.BeforeRouter, routers.PrometheusFilter)
	beego.InsertFilter("*", beego.BeforeRouter, routers.RecordMessage)
```

The introduction of `beego.InsertFilter` is as follows:

```
func InsertFilter(pattern string, pos int, filter FilterFunc, params ...bool) *App

InsertFilter adds a FilterFunc with pattern condition and action constant. The pos means action constant including beego.BeforeStatic, beego.BeforeRouter, beego.BeforeExec, beego.AfterExec and beego.FinishRouter. The bool params is for setting the returnOnOutput value (false allows multiple filters to execute)
```

When the `params` parameter is `false`, it runs multiple filters. The default is `true`.

So normally, if

```go
beego.InsertFilter("*", beego.BeforeRouter, routers.StaticFilter)
```

response something, the following filters will not be executed. But because the file does not exist, the function directly returns, causing the subsequent filters to continue executing. When it reaches

```go
beego.InsertFilter("*", beego.BeforeRouter, routers.ApiFilter)
```

it will start to check permissions:

```
subOwner = anonymous, subName = anonymous, method = GET, urlPath = /login, obj.Owner = , obj.Name = , result = deny
```

Then it will report this error:

```json
{
    "status": "error",
    "msg": "Unauthorized operation",
    "data": null,
    "data2": null
}
```

The solution should be:

```go
func StaticFilter(ctx *context.Context) {
	urlPath := ctx.Request.URL.Path

   /// omitted

	path := "web/build"
	if urlPath == "/" {
		path += "/index.html"
	} else {
		path += urlPath
	}

	if !util.FileExist(path) {
		// todo: response error: page not found
		return
	}

    /// omitted
}
```

* Update static_filter.go

---------

Co-authored-by: hsluoyz <hsluoyz@qq.com>

routers/static_filter.go

@@ -16,6 +16,7 @@ package routers
 
 import (
 	"compress/gzip"
+	"fmt"
 	"io"
 	"net/http"
 	"os"
@@ -59,6 +60,13 @@ func StaticFilter(ctx *context.Context) {
 		path = "web/build/index.html"
 	}
 	if !util.FileExist(path) {
+		dir, err := os.Getwd()
+		if err != nil {
+			panic(err)
+		}
+		dir = strings.ReplaceAll(dir, "\\", "/")
+		errorText := fmt.Sprintf("The Casdoor frontend HTML file: \"index.html\" was not found, it should be placed at: \"%s/web/build/index.html\". For more information, see: https://casdoor.org/docs/basic/server-installation/#frontend-1", dir)
+		http.ServeContent(ctx.ResponseWriter, ctx.Request, "Casdoor frontend has encountered error...", time.Now(), strings.NewReader(errorText))
 		return
 	}