Handle soroban loadgen mode misconfig gracefully #6354

Job	Run time
rust-check-git-rev-deps	16s
cargo-deny (advisories)	45s
fmt	16s
build (gcc, current)	1h 11m 0s
cargo-deny (bans licenses sources)	44s
build (gcc, current, --check-test-tx-meta)	57m 8s
build (gcc, next)	32m 15s
build (gcc, next, --check-test-tx-meta)	16m 13s
build (clang, current)	25m 6s
build (clang, current, --check-test-tx-meta)	15m 21s
build (clang, next)	24m 50s
build (clang, next, --check-test-tx-meta)	16m 10s
complete	1s
	4h 20m 5s

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