Make UpsertBucket less brittle

[JenGoldstrich]

Upcoming HCP SDK Go changes modified how we return the error code breaking this brittle check, I try to make it less brittle

The error goes from

{Code:5,"details":[],"message":"Error: The
bucket with identifier pkr-acctest-ancestry-parent does not exist."}

to

{"code":5,"details":[],"message":"Error: The
bucket with identifier pkr-acctest-ancestry-parent does not exist."}

Causing everything to break lol, this PR should work with both!

[lbajolet-hashicorp]

That's a weird change, the original error message was invalid JSON?
If this is now valid, we can probably even try to deserialise it instead of doing some string manipulation there, don't we?
I'm also surprised a SDK change is the cause for this behaviour too I would've thought the SDK passed through the response from the API instead of trying to re-format it.

Besides this I would think the code would work, though I would maybe go with a regex for this one, that would be likely simpler, and more optimal (even if performance is hardly a concern here)