Fixed assert failure when Soroban tx was sumbitted pre p20

[SirTyson]

Description

Following the TX frame refactor, there is an assertion failure when submitting Soroban TXs pre protocol 20. This was actually a bug before the refactor, where a Soroban TX would fail to get added to the queue with ADD_STATUS_ERROR, but the diagnostics would report txSUCCESS. The assert is correct, but the queue now rejects the transaction and sets txNOT_SUPPORTED. This is TX queue behavior so this won't effect TX replay.

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)
• Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document