allow providing an onError callable that is called on every error

Author: igorw

src/retry.php

@@ -4,12 +4,15 @@
 
 class FailingTooHardException extends \Exception {}
 
-function retry($retries, callable $fn)
+function retry($retries, callable $fn, callable $onError = null)
 {
     beginning:
     try {
         return $fn();
     } catch (\Exception $e) {
+        if ($onError) {
+            $onError($e);
+        }
         if (!$retries) {
             throw new FailingTooHardException('', 0, $e);
         }