Improve error handling on IPN

https://civicrm.stackexchange.com/questions/37277/paypal-standard-payments-are-being-accepted-but-marked-as-incomplete-transaction/37279#37279

shows how unhelpful this error is - getting data from the exception should help.

Targetting 5.28 in case the gitlab relates to a regression & we need to solicit more debug info

CRM/Core/Payment.php

@@ -1505,6 +1505,7 @@ public static function handleIPN() {
       Civi::log()->error('ipn_payment_callback_exception', [
         'context' => [
           'backtrace' => CRM_Core_Error::formatBacktrace(debug_backtrace()),
+          'message' => $e->getMessage(),
         ],
       ]);
     }