log deprecation events in Dark Console

Summary:
`PhutilErrorHandler` already defines "DEPRECATED" error class, but it's not used.
I plan to use it to report deprecation warnings.

With this change, they will be shown in the Dark Console and in the error logs

Ref T15554

This change on its own won't do anything, but is safe to land first.

Test Plan: With D25387, pages that have deprecation warnings are now shown, and the deprecation message is available in the dark console (and in log).

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15554

Differential Revision: https://we.phorge.it/D25386

src/applications/console/plugin/errorlog/DarkConsoleErrorLogPluginAPI.php

@@ -44,6 +44,7 @@ final class DarkConsoleErrorLogPluginAPI extends Phobject {
           'trace'     => $metadata['trace'],
         );
         break;
+      case PhutilErrorHandler::DEPRECATED:
       case PhutilErrorHandler::ERROR:
         // $value is a simple string
         self::$errors[] = array(