Resolve #3346676 "Dont requeue test mails"

Closes #3346676

src/Exception/MailgunClientException.php

+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\mailgun\Exception;
+
+/**
+ * Exception thrown when the Mailgun client encounters an error.
+ */
+class MailgunClientException extends \RuntimeException {
+
+  /**
+   * Create a new MailgunClientException from an exception.
+   *
+   * @param \Exception $e
+   *   The exception to create the MailgunClientException from.
+   * @param ?string $message
+   *   Optional message to use for the exception.
+   * @param ?int $code
+   *   Optional code to use for the exception.
+   *
+   * @return \Drupal\mailgun\Exception\MailgunClientException
+   *   The new MailgunClientException.
+   */
+  public static function fromException(\Exception $e, ?string $message = NULL, ?int $code = NULL): MailgunClientException {
+    $message = $message ?? $e->getMessage();
+    $code = $code ?? $e->getCode();
+
+    return new static($message, $code, $e);
+  }
+
+}