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 gohugoio#3817

docs/content/functions/errorf.md

@@ -0,0 +1,26 @@
+---
+title: errorf
+linktitle: errorf
+description: Evaluates a format string and logs it to ERROR.
+date: 2017-09-30
+publishdate: 2017-09-30
+lastmod: 2017-09-30
+categories: [functions]
+menu:
+  docs:
+    parent: "functions"
+keywords: [strings, log, error]
+signature: ["errorf FORMAT INPUT"]
+workson: []
+hugoversion:
+relatedfuncs: [printf]
+deprecated: false
+aliases: []
+---
+
+`errorf` will evaluate a format string, then output the result to the ERROR log.
+This will also cause the build to fail.
+
+```
+{{ errorf "Something went horribly wrong! %s" err }}
+```

tpl/fmt/fmt.go

@@ -15,6 +15,7 @@ package fmt
 
 import (
 	_fmt "fmt"
+	jww "github.com/spf13/jwalterweatherman"
 )
 
 // New returns a new instance of the fmt-namespaced template functions.
@@ -41,3 +42,8 @@ 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 {
+	jww.ERROR.Printf(format, a...)
+	return _fmt.Sprintf(format, a...)
+}

tpl/fmt/fmt_test.go

@@ -0,0 +1,14 @@
+package fmt
+
+import (
+	jww "github.com/spf13/jwalterweatherman"
+	"github.com/stretchr/testify/assert"
+	"testing"
+)
+
+func TestErrorf(t *testing.T) {
+	errorCount := jww.LogCountForLevel(jww.LevelError)
+	ns := &Namespace{}
+	assert.EqualValues(t, ns.Errorf("%s\n", "test error, please ignore."), "test error, please ignore.\n", "")
+	assert.EqualValues(t, errorCount+1, jww.LogCountForLevel(jww.LevelError), "Error log count not incremented")
+}

tpl/fmt/init.go

@@ -50,8 +50,14 @@ func init() {
 			},
 		)
 
-		return ns
+		ns.AddMethodMapping(ctx.Errorf,
+			[]string{"errorf"},
+			[][2]string{
+				{`{{ errorf "%s." "failed" }}`, `failed.`},
+			},
+		)
 
+		return ns
 	}
 
 	internal.AddTemplateFuncsNamespace(f)