fix: #242 ensuring alias is set or raise error.

[javydekoning]

Issue #, if available:
#242

Description of changes:

• Ensuring an error is raised if the account alias is non-unique.
• Update the readme.md to reflect that.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

Test execution (SHOULD error):

2022-04-03 13:17:47,547 \| ERROR \| configure_account_alias \| The account alias security already exists. The account alias must be unique across all Amazon Web Services products. Refer to https://docs.aws.amazon.com/IAM/latest/UserGuide/console_account-alias.html#AboutAccountAlias \| (configure_account_alias.py:26)
--
[ERROR] EntityAlreadyExistsException: An error occurred (EntityAlreadyExists) when calling the CreateAccountAlias operation: The account alias security already exists.Traceback (most recent call last):  File "/var/task/configure_account_alias.py", line 42, in lambda_handler    create_account_alias(event, role.client("iam"))  File "/var/task/configure_account_alias.py", line 30, in create_account_alias    raise error  File "/var/task/configure_account_alias.py", line 24, in create_account_alias    iam_client.create_account_alias(AccountAlias=account.get("alias"))  File "/var/task/botocore/client.py", line 401, in _api_call    return self._make_api_call(operation_name, kwargs)  File "/var/task/aws_xray_sdk/ext/botocore/patch.py", line 38, in _xray_traced_botocore    return xray_recorder.record_subsegment(  File "/var/task/aws_xray_sdk/core/recorder.py", line 435, in record_subsegment    return_value = wrapped(*args, **kwargs)  File "/var/task/botocore/client.py", line 731, in _make_api_call    raise error_class(parsed_response, operation_name)

[StewartW]

@javydekoning Can you add a test for this exception too please

[javydekoning]

@StewartW yes, I've added a test. Thanks for reviewing!

[sbkok]

LGTM, thank you!