Output a clearer error message for detect failures

[edmorley]

Currently if bin/detect for this buildpack fails to find a package.json, the build outputs shows:

$ mkdir -p testapp-cra && cd $_ && git init && h create
$ h buildpacks:add mars/create-react-app
$ touch foo
$ git add -A && git commit -m '.' && git push heroku main
...
remote: -----> App not compatible with buildpack: https://buildpack-registry.s3.amazonaws.com/buildpacks/mars/create-react-app.tgz
remote:        More info: https://devcenter.heroku.com/articles/buildpacks#detection-failure
remote:
remote:  !     Push failed

If bin/detect were to instead output a package.json not found in app source type error message to stderr (must be stderr or the build system ignores it), then that would be shown in the output instead.

Examples of what other buildpacks output:
https://github.com/heroku/heroku-buildpack-nodejs/blob/master/bin/detect
https://github.com/heroku/heroku-buildpack-php/blob/main/bin/detect