Group template helper functions, remove Printf, improve template error messages (#23982)

Follow #23328 


Major changes:

* Group the function in `templates/help.go` by their purposes. It could
make future work easier.
* Remove the `Printf` helper function, there is already a builtin
`printf`.
* Remove `DiffStatsWidth`, replace with `Eval` in template
* Rename the `NewTextFuncMap` to `mailSubjectTextFuncMap`, it's for
subject text template only, no need to make it support HTML functions.


----

And fine tune template error messages, to make it more friendly to
developers and users.


![image](https://user-images.githubusercontent.com/2114189/230714245-4fd202d1-2b25-41b2-8be5-03c5fee45091.png)


![image](https://user-images.githubusercontent.com/2114189/230714277-66783577-2a03-49d5-8e8c-ceba5e07a2d4.png)

---------

Co-authored-by: silverwind <me@silverwind.io>

routers/web/swagger_json.go

@@ -4,11 +4,8 @@
 package web
 
 import (
-	"net/http"
-
 	"code.gitea.io/gitea/modules/base"
 	"code.gitea.io/gitea/modules/context"
-	"code.gitea.io/gitea/modules/log"
 )
 
 // tplSwaggerV1Json swagger v1 json template
@@ -16,10 +13,13 @@ const tplSwaggerV1Json base.TplName = "swagger/v1_json"
 
 // SwaggerV1Json render swagger v1 json
 func SwaggerV1Json(ctx *context.Context) {
-	t := ctx.Render.TemplateLookup(string(tplSwaggerV1Json))
+	t, err := ctx.Render.TemplateLookup(string(tplSwaggerV1Json))
+	if err != nil {
+		ctx.ServerError("unable to find template", err)
+		return
+	}
 	ctx.Resp.Header().Set("Content-Type", "application/json")
-	if err := t.Execute(ctx.Resp, ctx.Data); err != nil {
-		log.Error("%v", err)
-		ctx.Error(http.StatusInternalServerError)
+	if err = t.Execute(ctx.Resp, ctx.Data); err != nil {
+		ctx.ServerError("unable to execute template", err)
 	}
 }