Improve RunMode / dev mode (#24886)

1. non-dev mode is treated as prod mode, to protect users from
accidentally running in dev mode if there is a typo in this value.
2. in dev mode, do not need to really exit if there are template errors,
because the template errors could be fixed by developer soon and the
templates get reloaded, help:
* https://github.com/go-gitea/gitea/issues/24845#issuecomment-1557615382
3. Fine tune the mail template loading message.

modules/templates/mailer.go

@@ -61,7 +61,10 @@ func Mailer(ctx context.Context) (*texttmpl.Template, *template.Template) {
 	bodyTemplates.Funcs(NewFuncMap())
 
 	assetFS := AssetFS()
-	refreshTemplates := func() {
+	refreshTemplates := func(firstRun bool) {
+		if !firstRun {
+			log.Trace("Reloading mail templates")
+		}
 		assetPaths, err := ListMailTemplateAssetNames(assetFS)
 		if err != nil {
 			log.Error("Failed to list mail templates: %v", err)
@@ -75,17 +78,21 @@ func Mailer(ctx context.Context) (*texttmpl.Template, *template.Template) {
 				continue
 			}
 			tmplName := strings.TrimPrefix(strings.TrimSuffix(assetPath, ".tmpl"), "mail/")
-			log.Trace("Adding mail template %s: %s by %s", tmplName, assetPath, layerName)
+			if firstRun {
+				log.Trace("Adding mail template %s: %s by %s", tmplName, assetPath, layerName)
+			}
 			buildSubjectBodyTemplate(subjectTemplates, bodyTemplates, tmplName, content)
 		}
 	}
 
-	refreshTemplates()
+	refreshTemplates(true)
 
 	if !setting.IsProd {
 		// Now subjectTemplates and bodyTemplates are both synchronized
 		// thus it is safe to call refresh from a different goroutine
-		go assetFS.WatchLocalChanges(ctx, refreshTemplates)
+		go assetFS.WatchLocalChanges(ctx, func() {
+			refreshTemplates(false)
+		})
 	}
 
 	return subjectTemplates, bodyTemplates