tpl: Do not return any value in errorf

Fixes #6653

docs/content/en/functions/errorf.md

@@ -14,14 +14,14 @@ workson: []
 hugoversion:
 relatedfuncs: [printf]
 deprecated: false
-aliases: []
+aliases: [/functions/errorf]
 ---
 
 `errorf` or `warnf` will evaluate a format string, then output the result to the ERROR or WARNING log (and only once per error message to avoid flooding the log).
 
 Any ERROR will also cause the build to fail (the `hugo` command will `exit -1`).
 
-Note that the WARNING will only be printed to the console.
+Both functions return an empty string, so the messages are only printed to the console.
 
 ```
 {{ errorf "Failed to handle page %q" .Path }}

tpl/fmt/fmt.go

@@ -51,11 +51,11 @@ func (ns *Namespace) Println(a ...interface{}) string {
 	return _fmt.Sprintln(a...)
 }
 
-// Errorf formats according to a format specifier and returns the string as a
-// value that satisfies error.
+// Errorf formats according to a format specifier and logs an ERROR.
+// It returns an empty string.
 func (ns *Namespace) Errorf(format string, a ...interface{}) string {
 	ns.errorLogger.Printf(format, a...)
-	return _fmt.Sprintf(format, a...)
+	return ""
 }
 
 // Warnf formats according to a format specifier and logs a WARNING.

tpl/fmt/init.go

@@ -53,7 +53,7 @@ func init() {
 		ns.AddMethodMapping(ctx.Errorf,
 			[]string{"errorf"},
 			[][2]string{
-				{`{{ errorf "%s." "failed" }}`, `failed.`},
+				{`{{ errorf "%s." "failed" }}`, ``},
 			},
 		)