fixed create/delete handler pre execution check

[ammokhov]

Issue #, if available: #265

Description of changes:

• Refactoring crud handler stubs so it's less verbose and more one-liner.
• CreateHandler and DeleteHandler have fixed pre execution checks which add on the main call graph - codegen - create/delete handler pre execution check  #265

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[miparnisari]

I tested throwing exceptions here, and they are not bubbled all the way up. The handler ends up returning a status code of FAILED but no exception is thrown.

[rjlohan]

The intent is the Handler should return FAILED with a sane/consistent exception message that ends up in Stack Trace. I think there's a separate issue that needs to be fixed for that as it's not working reliably.

[miparnisari]

@rjlohan So which of these asserts should I use?

assertThatThrownBy(() -> handler.handleRequest(proxy, request, new CallbackContext(), proxyClient, logger))
 .isInstanceOf(CfnAlreadyExistsException.class);

or

response = handler.handleRequest(proxy, request, new CallbackContext(), proxyClient, logger);
assertThat(response).isNotNull();
assertThat(response.getStatus()).isEqualTo(OperationStatus.FAILED);
assertThat(response.getErrorCode()).isEqualTo(HandlerErrorCode.AlreadyExists);

I've seen plenty of examples of the former but according to your suggestion we should use the latter?