[milo] Refactor all html knowledge out of backends.

milo/frontend/view_error.go

Index: milo/frontend/view_error.go
diff --git a/milo/frontend/view_error.go b/milo/frontend/view_error.go
new file mode 100644
index 0000000000000000000000000000000000000000..57afe71379b8d597132a36284d2fbb2a658f91c9
--- /dev/null
+++ b/milo/frontend/view_error.go
@@ -0,0 +1,31 @@
+// Copyright 2017 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package frontend
+
+import (
+	"github.com/luci/luci-go/common/errors"
+	"github.com/luci/luci-go/server/router"
+	"github.com/luci/luci-go/server/templates"
+
+	"github.com/luci/luci-go/milo/common"
+)
+
+// ErrorHandler renders an error page for the user.
+func ErrorHandler(c *router.Context, err error) {
+	// TODO(iannucci): tag/extract other information from error, like a link to the
+	// 'container'; i.e. a build may link to its builder, a builder to its
+	// master/bucket, etc.
+
+	code := common.ErrorTag.In(err)
+	if code == common.CodeUnknown {
+		errors.Log(c.Context, err)
+	}
+	status := code.HTTPStatus()
+	c.Writer.WriteHeader(status)
+	templates.MustRender(c.Context, c.Writer, "pages/error.html", templates.Args{
+		"Code":    status,
+		"Message": err.Error(),
+	})
+}