tools/serve-crustgather-artifact.sh: fail early (e.g. if bad archive)

This is a simple addition of set -e in tools/serve-crustgather-artifact.sh to let it fail early, for instance if the archive is bad.

The case I ran into was:

./tools/serve-crustgather-artifact.sh -j 8019652726                                  
Downloading from https://gitlab.com/api/v4/projects/42451983/jobs/8019652726/artifacts/crust-gather.tar.gz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100    27  100    27    0     0    102      0 --:--:-- --:--:-- --:--:--   102

gzip: stdin: not in gzip format
tar: Child returned status 1
tar: Error is not recoverable: exiting now
Serving on 127.0.0.1:9385...
2024-10-08T11:48:55.940622Z  INFO actix_server::builder: starting 8 workers
2024-10-08T11:48:55.940644Z  INFO actix_server::server: Tokio runtime found; starting in existing Tokio runtime