fix: IAM Policy 409 concurrent changes error - take 2

[modular-magician]

Fixes hashicorp/terraform-provider-google#25305

There is already a backoff retry mechanism but it's broken due to wrapped error. The current usage for checking if it's googleapi error doesn't work well.

#15972 reverted #15825 due to regressions. However, the original IAM 409 concurrent error is now back due to that.

I debugged that specific error and found that the err var returned is by the type fmt.Errorf on level 0 and googleapi.Error on level 1.
errwrap simply doesn't detect it.

I wanted to avoid walking or unwrapping and make minimal changes as possible.

Given the fact we have to keep using errwrap for now, I added the same changes I did originally below the current errwrap code and also I added tests.

I tested the generated provider and it solves the 409 issue.

Tagging @slevenick @ScottSuarez @BBBmau for extra verification and input.

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iam: fixed error 409 concurrency policy changes by correctly detecting the error type.

Derived from GoogleCloudPlatform/magic-modules#15995