[milo] Refactor all html knowledge out of backends.

milo/frontend/view_error.go

+// 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(),
+        })
+}