Fail load gen if soroban invoke without setup

[ThomasBrady]

https://github.com/stellar/stellar-core/pull/4559 changed load gen to throw an exception with an error message if a user generated load in MODE::SOROBAN_INVOKE mode, without first running MODE::SOROBAN_INVOKE_SETUP.

This PR reverts that change and instead logs an error message and calls emitFailure to increment loadgen.run.failed and reset the state. It tests that this metric is incremented in this case.

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