do error message pass (especially for generic/js)

[ashleygwilliams]

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.

[ashleygwilliams]

this error message needs to say something about adding the dist.js and npm scripts

[ashleygwilliams]

(also we should not panic, definitely should be a handled error)