Show 404 when remote user not found

This notifies the user that the remote user doesn't exist, instead of
showing a blank page.

Ref T627

collections.go

@@ -862,9 +862,9 @@ func handleViewMention(app *App, w http.ResponseWriter, r *http.Request) error {
 	handle := vars["handle"]
 
 	remoteUser, err := app.db.GetProfilePageFromHandle(app, handle)
-	if err != nil {
-		log.Error("Couldn't find this user "+handle, err)
-		return nil
+	if err != nil || remoteUser == "" {
+		log.Error("Couldn't find user %s: %v", handle, err)
+		return ErrRemoteUserNotFound
 	}
 
 	return impart.HTTPError{Status: http.StatusFound, Message: remoteUser}

errors.go

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2018 A Bunch Tell LLC.
+ * Copyright © 2018-2020 A Bunch Tell LLC.
  *
  * This file is part of WriteFreely.
  *
@@ -46,6 +46,7 @@ var (
 	ErrPostFetchError         = impart.HTTPError{Status: http.StatusInternalServerError, Message: "We encountered an error getting the post. The humans have been alerted."}
 
 	ErrUserNotFound       = impart.HTTPError{http.StatusNotFound, "User doesn't exist."}
+	ErrRemoteUserNotFound = impart.HTTPError{http.StatusNotFound, "Remote user not found."}
 	ErrUserNotFoundEmail  = impart.HTTPError{http.StatusNotFound, "Please enter your username instead of your email address."}
 
 	ErrUserSuspended = impart.HTTPError{http.StatusForbidden, "Account is silenced."}