Add a way to merge pages by language

As an example:

```html
{{ $pages := .Site.RegularPages | lang.Merge $frSite.RegularPages | lang.Merge $enSite.RegularPages }}
```

Will "fill in the gaps" in the current site with, from left to right, content from the French site, and lastly the English.

Fixes #4463

hugolib/hugo_sites_build.go

@@ -15,9 +15,12 @@ package hugolib
 
 import (
 	"bytes"
+	"fmt"
 
 	"errors"
 
+	jww "github.com/spf13/jwalterweatherman"
+
 	"github.com/fsnotify/fsnotify"
 	"github.com/gohugoio/hugo/helpers"
 )
@@ -71,6 +74,11 @@ func (h *HugoSites) Build(config BuildCfg, events ...fsnotify.Event) error {
 		h.Log.FEEDBACK.Println()
 	}
 
+	errorCount := h.Log.LogCountForLevel(jww.LevelError)
+	if errorCount > 0 {
+		return fmt.Errorf("logged %d error(s)", errorCount)
+	}
+
 	return nil
 
 }