Better errors when the lean4export version is incorrect

Lean4export includes a header with the version and this version (must? usually needs to?) exactly match the precise version of Lean being used. I got stuck for awhile because I had lean4export for 4.29.0-rc7 on my PATH, unbeknownst to me and with higher priority than the 4.30.0-rc2 lean4export I'd added to the PATH as well, so comparator 4.30.0-rc2 was failing unhelpfully.

Building Challenge
⚠ [2/3] Built Challenge
warning: Challenge.lean:1:8: declaration uses `sorry`
Build completed successfully (3 jobs).
Exporting #[amazing, propext, Quot.sound, Classical.choice, Nat.add, Nat.sub, Nat.mul, Nat.pow, Nat.gcd, Nat.div, Nat.mod, Nat.beq, Nat.ble, Nat.land, Nat.lor, Nat.xor, Nat.shiftLeft, Nat.shiftRight, String.ofList] from Challenge
uncaught exception: process 'landrun' exited with code 1
stderr:
uncaught exception: process 'lean' exited with code 255
stderr:
could not execute external process 'lean'

No source file or test is named in the issue. Start by tracing the comparator path that invokes lean4export and reproduce the mismatch between the 4.29.0-rc7 and 4.30.0-rc2 binaries; done means the version mismatch is reported clearly instead of producing the shown process failure.