hugolib: Return error from .Render when template not found

Closes #15052

hugolib/page__per_output.go

@@ -114,7 +114,7 @@ func (pco *pageContentOutput) Render(ctx context.Context, layout ...string) (tem
 	}
 
 	if !found {
-		return "", nil
+		return "", fmt.Errorf("template %q not found", layout[0])
 	}
 
 	// Make sure to send the *pageState and not the *pageContentOutput to the template.

hugolib/page_test.go

@@ -2055,3 +2055,21 @@ disableKinds = ["taxonomy", "term"]
 		}
 	}
 }
+
+// See issue 15052.
+func TestRenderViewNotFound(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+disableKinds = ['page','rss','section','sitemap','taxonomy','term']
+-- layouts/home.html --
+{{ .Render "view_foo" }}
+{{ .Render "view_bar" }}
+-- layouts/view_foo.html --
+foo
+`
+
+	b, err := TestE(t, files)
+	b.Assert(err, qt.ErrorMatches, `.*template "view_bar" not found.*`)
+}