Handle soroban loadgen mode misconfig gracefully #6369

Job	Run time
fmt	16s
rust-check-git-rev-deps	19s
build (gcc, current)	30m 46s
cargo-deny (advisories)	42s
build (gcc, current, --check-test-tx-meta)	17m 48s
cargo-deny (bans licenses sources)	42s
build (gcc, next)	28m 17s
build (gcc, next, --check-test-tx-meta)	16m 18s
build (clang, current)	25m 27s
build (clang, current, --check-test-tx-meta)	15m 12s
build (clang, next)	25m 47s
build (clang, next, --check-test-tx-meta)	16m 45s
complete	1s
	2h 58m 20s

Provide feedback