azblob: wrong example in error casting for StorageError

[bcho]

Bug Report

• import path of package in question:.../sdk/storage/azblob
• SDK version: main
• output of go version: go version go1.17.3 darwin/amd64

• What happened?

In zt_examples_test, the doc states that we can extract the StorageError from response as:

azure-sdk-for-go/sdk/storage/azblob/zt_examples_test.go

Lines 183 to 187 in 916bcb2

	var stgErr StorageError
	if errors.As(err, &stgErr) { // We know this error is service-specific
	switch stgErr.ErrorCode {
	case StorageErrorCodeContainerAlreadyExists:

However, this is not valid in current implementation, since we are returning the StorageError with pointer value in handleError:

azure-sdk-for-go/sdk/storage/azblob/zc_storage_error.go

Lines 62 to 65 in 916bcb2

	func handleError(err error) error {
	var respErr *azcore.ResponseError
	if errors.As(err, &respErr) {
	return &InternalError{responseErrorToStorageError(respErr)}

I am also confused that, why we are declaring the StorageError struct itself as an error, but returning a pointer in handleError? Why not just declare the *StorageError as error and pass this pointer around?

• What did you expect or want to happen?

We should update the doc (but IMO updating the error interface for StorageError is better) to:

		var stgErr *StorageError

		if errors.As(err, &stgErr) { // We know this error is service-specific
			switch stgErr.ErrorCode {
			case StorageErrorCodeContainerAlreadyExists:

• How can we reproduce it?

internalErr := &InternalError{
  cause: &StorageError{},
}

{
    var target StorageError
    fmt.Println(errors.As(internalErr, &target))
}

{
    var target *StorageError
    fmt.Println(errors.As(internalErr, &target))
}

• Anything we should know about your environment.

[jhendrixMSFT]

Failed operations now return an *azcore.ResponseError, and there is a helper bloberror.HasCode() to check for specific storage error codes.