tpl: Add errorf template function

Add template function that will build a string from the given format
string and arguments, then log it to ERROR. This has an intended
side-effect of causing the build to fail, when executed.

Resolves #3817

tpl/fmt/fmt.go

@@ -15,15 +15,17 @@ package fmt
 
 import (
 	_fmt "fmt"
+	"github.com/gohugoio/hugo/helpers"
 )
 
 // New returns a new instance of the fmt-namespaced template functions.
 func New() *Namespace {
-	return &Namespace{}
+	return &Namespace{helpers.NewDistinctErrorLogger()}
 }
 
 // Namespace provides template functions for the "fmt" namespace.
 type Namespace struct {
+	errorLogger *helpers.DistinctLogger
 }
 
 // Print returns string representation of the passed arguments.
@@ -41,3 +43,8 @@ func (ns *Namespace) Printf(format string, a ...interface{}) string {
 func (ns *Namespace) Println(a ...interface{}) string {
 	return _fmt.Sprintln(a...)
 }
+
+func (ns *Namespace) Errorf(format string, a ...interface{}) string {
+	ns.errorLogger.Printf(format, a...)
+	return _fmt.Sprintf(format, a...)
+}