Checks if the input dictionary of users is valid.

def _check_users(users):

    '''Checks if the input dictionary of users is valid.'''

    messg = ''
    valid = True

    for user, user_details in six.iteritems(users):
        if not user_details:
            valid = False
            messg += 'Please provide details for username {user}.\n'.format(user=user)
            continue
        if not (isinstance(user_details.get('level'), int) or 0 <= user_details.get('level') <= 15):
            # warn!
            messg += 'Level must be a integer between 0 and 15 for username {user}. Will assume 0.\n'.format(user=user)

    return valid, messg