Set HTTP response code to 500 in case of exceptions

Resolves Seldaek/monolog#1096.

src/Monolog/ErrorHandler.php

@@ -166,6 +166,10 @@ class ErrorHandler
             call_user_func($this->previousExceptionHandler, $e);
         }
 
+        if (!headers_sent() && ini_get('display_errors') === 0) {
+            http_response_code(500);
+        }
+
         exit(255);
     }