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>

1 files changed, 4 insertions, 4 deletions

diff --git a/modules/context/context.go b/modules/context/context.go
index 04f8a9bd3d..bd561be0f5 100644
--- a/modules/context/context.go
+++ b/modules/context/context.go
@@ -47,7 +47,7 @@ import (
 
 // Render represents a template render
 type Render interface {
-	TemplateLookup(tmpl string) *template.Template
+	TemplateLookup(tmpl string) (*template.Template, error)
 	HTML(w io.Writer, status int, name string, data interface{}) error
 }
 
@@ -228,7 +228,7 @@ func (ctx *Context) HTML(status int, name base.TplName) {
 	}
 	if err := ctx.Render.HTML(ctx.Resp, status, string(name), templates.BaseVars().Merge(ctx.Data)); err != nil {
 		if status == http.StatusInternalServerError && name == base.TplName("status/500") {
-			ctx.PlainText(http.StatusInternalServerError, "Unable to find status/500 template")
+			ctx.PlainText(http.StatusInternalServerError, "Unable to find HTML templates, the template system is not initialized, or Gitea can't find your template files.")
 			return
 		}
 		if execErr, ok := err.(texttemplate.ExecError); ok {
@@ -247,7 +247,7 @@ func (ctx *Context) HTML(status int, name base.TplName) {
 				if errorTemplateName != string(name) {
 					filename += " (subtemplate of " + string(name) + ")"
 				}
-				err = fmt.Errorf("%w\nin template file %s:\n%s", err, filename, templates.GetLineFromTemplate(errorTemplateName, line, target, pos))
+				err = fmt.Errorf("failed to render %s, error: %w:\n%s", filename, err, templates.GetLineFromTemplate(errorTemplateName, line, target, pos))
 			} else {
 				filename, filenameErr := templates.GetAssetFilename("templates/" + execErr.Name + ".tmpl")
 				if filenameErr != nil {
@@ -256,7 +256,7 @@ func (ctx *Context) HTML(status int, name base.TplName) {
 				if execErr.Name != string(name) {
 					filename += " (subtemplate of " + string(name) + ")"
 				}
-				err = fmt.Errorf("%w\nin template file %s", err, filename)
+				err = fmt.Errorf("failed to render %s, error: %w", filename, err)
 			}
 		}
 		ctx.ServerError("Render failed", err)