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

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 }}
+```