[Backport 2.3-develop] Handle multiple errors in customer address validation when shown in adminhtml customer edit page

[adrian-martinez-interactiv4]

Description

When multiple validation errors are found trying to save a customer address, errors are not properly shown in adminhtml customer edit page. Related with PR #12922

Fixed Issues (if relevant)

None AFAIK

Manual testing scenarios

1. Simulate several validation errors when trying to save a customer address from admin, for example, adding temporarily this code almost at the end of \Magento\Customer\Model\Address\AbstractAddress::validate method:

    /**
     * Validate address attribute values
     *
     * @return bool|array
     * @SuppressWarnings(PHPMD.CyclomaticComplexity)
     * @SuppressWarnings(PHPMD.NPathComplexity)
     */
    public function validate()
    {
        (...)

        $errors = [];
        $errors[] = __('Error 1');
        $errors[] = __('Error 2');

        if (empty($errors) || $this->getShouldIgnoreValidation()) {
            return true;
        }
        return $errors;
    }

Expected result

Actual result

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)