eth/api_backend: return error code for PoS tag requests before merge #28309
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There is a specific testcase in hive that verifies the error code of PoS tag requests ("safe" / "finalized") before the finalization of the merge. This PR fixes that test.
Personally, I think the test is a bit overzealous and should be happy so long as the request errors. This is the first specific error code we return in the
ethJSON-RPC as far as I know. My preference would be to close this PR and update Hive instead with this: ethereum/hive#874