Make build scripts exit when a command fails

[PEliet]

Summary

• Make build.sh exit when a normal command fails.
• Apply the same failure handling to build-web.sh.
• Correct the comments to describe both ordinary-command and pipeline failures.

set -o pipefail only changes how a pipeline reports failure; it does not stop a script after a standalone command fails. Adding set -e ensures a failure from commands such as lake exe cache get is propagated instead of allowing the script to continue.

Testing

• Not run locally: this change only affects Bash startup options, and the current Windows environment has no usable Bash/WSL distribution.
• Please rely on the repository's GitHub Actions build for full validation.