Need bettter error when runtime fails to execve the process

[deciduously]

I hit a case where we had not made the symlink to sh available in the artifacts directory before trying to run it. Instead of failing, I saw an ENOENT in the logs for the execve, but ultimately the JS parent that spawned this child continued running, eventually failing with an "uncaught undefined". Here is a log snippet:

[runner]:   2025-03-05T18:32:14.582320Z ERROR tangram_server::runtime::linux::process: Cannot access path: /.tangram/artifacts/sym_01xpnr55xrsjcwcc9ppryzqry6r2m15k17kzjxjakyfs4g5fvksqqg (errno=2): No such file or directory (os error 2)
[runner]:     at packages/server/src/runtime/linux/process.rs:527
[runner]:   2025-03-05T18:32:14.582357Z DEBUG tangram_server::runtime::linux::process: about to execve, exec: "/.tangram/artifacts/sym_01xpnr55xrsjcwcc9ppryzqry6r2m15k17kzjxjakyfs4g5fvksqqg"
[runner]:     at packages/server/src/runtime/linux/process.rs:538
[runner]: an error occurred in the linux runtime guest process
[runner]: failed to call execve
[runner]: No such file or directory (os error 2)
[server]:   2025-03-05T18:32:14.630428Z DEBUG tangram_server::store::lmdb: try_get start
[server]:     at packages/server/src/store/lmdb.rs:89
[runner]:   2025-03-05T18:32:14.679815Z DEBUG tangram_server::runtime::js: finished js, result: Err(Error { code: None, message: Some("Uncaught undefined"), location: None, stack: None, source: None, values: {} })
[runner]:     at packages/server/src/runtime/js.rs:383

This code shows the runner logging the execve failure, but then the JS process finishing with the uncaught undefined. This execve failure should have halted the JS process as well, and reported a better error back up to my client.