do error message pass (especially for generic/js)

repro

1. add dist-workspace.toml
2. run cargo dist init

```
✔ added [dist] to your root dist-workspace.toml
✔ cargo-dist is setup!

running 'cargo dist generate' to apply any changes

× cargo dist panicked.
├─▶ at cargo-dist/src/build/generic.rs:59:26
╰─▶ A build command is mandatory for generic builds
help: set the `RUST_BACKTRACE=1` environment variable to display a backtrace.
```

Reproduce the failure with dist-workspace.toml, cargo dist init, and cargo dist generate. Start at cargo-dist/src/build/generic.rs:59:26, then trace the generic and JavaScript error paths; done means the error messages no longer surface an avoidable panic and the reported guidance is clear.

Written by the indexing model from the issue text.