Improve UI messages

Right now we have these messages that are displayed to consumers:
> Betaling wordt a-sync verwerkt. @todo: maak deze tekst begrijpbaar voor consumenten.
> You have returned checkout at @gateway. We will inform you when you payment has been validated.

  /**
   * Builds the payment instructions.
   *
   * @param \Drupal\commerce_payment\Entity\PaymentInterface $payment
   *   The payment.
   *
   * @return array
   *   A render array containing the payment instructions.
   */
  public function buildPaymentInstructions(PaymentInterface $payment = NULL) {
    $instructions = [
      '#type' => 'processed_text',
      '#text' => 'Betaling wordt a-sync verwerkt. @todo: maak deze tekst begrijpbaar voor consumenten.',
      '#format' => 'plain_text',
    ];

  /**
   * {@inheritdoc}
   */
  public function onReturn(OrderInterface $order, Request $request) {
    drupal_set_message($this->t('You have returned checkout at @gateway. We will inform you when you payment has been validated.', [
      '@gateway' => $this->getDisplayLabel(),
    ]));
  }

We should also add a $context to the t() function and always use the t() function.
Example from address module:

t('Street address', [], ['context' => 'Address label']),
t('First name', [], ['context' => 'Address label']),
t('Last name', [], ['context' => 'Address label']),

Maybe context could be 'Commerce Mollie message'