def ValidateSizeQuota(value):
    """
    Validate a quota value.  This may be blank or a non-negative integer.

    :param value: The proposed value.
    :type value: int
    :returns: The validated value or None,
              and a recommended error message or None.
    :rtype: (int or None, str or None)
    """
    if value is None or value == '':
        return None, None
    error = False
    try:
        value = int(value)
        if value < 0:
            error = True
    except ValueError:
        error = True
    if error:
        return (value,
            'Invalid quota.  Must be blank or a non-negative integer representing the limit in bytes.'
            )
    return value, None
