[TC][OBJSTORE] Use defined error rather than string when checking return of GetSASURL

What is the problem?

In MB-55201 we upgraded the Azure SDK. Unfortunately they removed the ability to check if a client was created with a shared key, which we need to know whether we should call GetSASURL or just URL. Currently we hardcode the error string and check that, but this is brittle.

What is the fix?
When we next upgrade the Azure SDK we should use the exported error they are introducing (https://github.com/Azure/azure-sdk-for-go/issues/19864)