Fix for async deletion of non-existent resource

[EronWright]

This PR fixes an error that occurs in the deletion of a (non-existent) Azure resource. The issue impacts resources that have an async-style DELETE call (as is the case for Subnet, see schema).

The user-facing error is:

  azure-native:network:Subnet (subnet):
    error: the operation failed or was cancelled

The call to runtime.NewPoller should not be made with a failed response, otherwise we hit a precondition within it:

	// this is a back-stop in case the swagger is incorrect (i.e. missing one or more status codes for success).
	// ideally the codegen should return an error if the initial response failed and not even create a poller.
	if !poller.StatusCodeValid(resp) {
		return nil, errors.New("the operation failed or was cancelled")
	}

To fix, I encapsulated the request-to-final-response logic into an anonymous function, to be able to uniformly check for the 404 result. And I ensure that the initial response was successful before making a poller.

This PR relaxes the logic for detecting a 404 during resource deletion, to not care about the specific error code. I find that at least two codes are possible: ResourceNotFound and ResourceGroupNotFound. Note that the sync path did not check the error code.

TODO:

• new unit tests
• manual testing for various types of resources (w/ sync and async delete operations).

Closes #3978
See also: #3783

[github-actions]

Does the PR have any schema changes?

Looking good! No breaking changes found.
No new resources/functions.

[EronWright]

@thomas11 , I notice that normalizeLocationHeader isn't being called here, as is in putOrPatch.
Given this discussion, it probably doesn't matter, but I bring it up in case you'd like me to act on that.

[EronWright]

This fix is actually a follow-up to #3783. I think it is justifiably in here because the Delete call is being similarly fixed here.

[EronWright]

When we fake an error response, it is good to use the error schema because you can make assertions about the final error code.

[codecov]

Codecov Report

Attention: Patch coverage is 73.91304% with 6 lines in your changes missing coverage. Please review.

Project coverage is 57.49%. Comparing base (73833a7) to head (8ef8cbc).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
provider/pkg/azure/client_azcore.go	73.91%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3980   +/-   ##
=======================================
  Coverage   57.49%   57.49%           
=======================================
  Files          82       82           
  Lines       13078    13077    -1     
=======================================
  Hits         7519     7519           
+ Misses       4984     4983    -1     
  Partials      575      575           

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