Read canister id from 500 status response

[lmuntaner]

Yesterday in a debugging session with Civic I realized that the problem was that they had as a derivation origin a custom domain pointing to a backend canister that was only serving the alternative domains path. Therefore, II couldn't resolve the canister id because it kept getting a 500 error in the HEAD request.

I asked them to change the derivation origin to the canister id url .icp0.io to unblock them.

Even though it might be an edge case to have a derivation origin a backend canister with a custom domain, I don't see why we shouldn't support it.

Conclusion: I removed the status check from the response to read the canister id from the headers.

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