Force reporting of E_USER_* in error api

Solves the problem of Mantis silently proceeding through errors that
should effectively stop code execution, when error_reporting is disabled
in php.ini or apache mod config.

This follows @davidhicks recommendation in comment ~22998.

Fixes #10966

Backported from master cec4549.

core/error_api.php

@@ -27,6 +27,10 @@
 $g_error_proceed_url = null;
 $g_error_send_page_header = true;
 
+# Make sure we always capture User-defined errors regardless of ini settings
+# These can be disabled in config_inc.php, see $g_display_errors
+error_reporting( error_reporting() | E_USER_ERROR | E_USER_WARNING | E_USER_NOTICE );
+
 set_error_handler( 'error_handler' );
 
 /**