Milo: Add themed page for errors

milo/appengine/buildbot/build.go

Index: milo/appengine/buildbot/build.go
diff --git a/milo/appengine/buildbot/build.go b/milo/appengine/buildbot/build.go
index da6149f1ba18dbb590abe2867e611399511bdffb..f021ea0727fed9c0bfffcb3bc156ff2e70b431b8 100644
--- a/milo/appengine/buildbot/build.go
+++ b/milo/appengine/buildbot/build.go
@@ -20,6 +20,8 @@ import (
 	"github.com/luci/luci-go/milo/api/resp"
 	"github.com/luci/luci-go/milo/appengine/settings"
 	"github.com/luci/luci-go/milo/common/miloerror"
+	"github.com/luci/luci-go/server/auth"
+	"github.com/luci/luci-go/server/auth/identity"
 	"golang.org/x/net/context"
 )
 
@@ -29,17 +31,30 @@ var errBuildNotFound = miloerror.Error{
 }
 
 // getBuild fetches a buildbot build from the datastore and checks ACLs.
+// The return code matches the master responses, that is:
+// User not logged in, master found, master public: 200
+// User not logged in, master not found: 401
+// User not logged in, master internal: 401
+// User logged in, master found, master internal: 200
+// User logged in, master not found: 404
+// User logged in, master found, master internal: 404
+// Other error: 500
 func getBuild(c context.Context, master, builder string, buildNum int) (*buildbotBuild, error) {
 	result := &buildbotBuild{
 		Master:      master,
 		Buildername: builder,
 		Number:      buildNum,
 	}
+	cu := auth.CurrentUser(c)
 
 	err := ds.Get(c, result)
 	switch {
 	case err == ds.ErrNoSuchEntity:
-		return nil, errBuildNotFound
+		if cu.Identity == identity.AnonymousIdentity {
+			return nil, errNotAuth
+		} else {
+			return nil, errBuildNotFound
+		}
 	case err != nil:
 		return nil, err
 	}
@@ -49,7 +64,11 @@ func getBuild(c context.Context, master, builder string, buildNum int) (*buildbo
 			return nil, err
 		}
 		if !allowed {
-			return nil, errBuildNotFound
+			if cu.Identity == identity.AnonymousIdentity {
+				return nil, errNotAuth
+			} else {
+				return nil, errBuildNotFound
+			}
 		}
 	}