sbt-rjs fails without packaging failing

[paddymahoney]

With sbt-web 1.1.1, rjs 1.0.7, and
pipelineStages := Seq(rjs, digest, gzip),
and no main.js, the result of packaging is that there is an error in the console, but a package is produced. Assets are subsequently broken when the resulting package is deployed.

The issue is that without a main.js, we get a message to console:
[info] Error: Error: /home/patrick/npl/auvik/target/web/rjs/appdir/javascripts/main.js does not exist.

It may be better if the packaging failed immediately, as the assets don't display once installed.

Sidenote: if the support is there within sbt-web's js engine, it might help if a similar log output format is used - some build tools might miss Error: Error: while they might be able to report [error] as a task failure when packaging.

Thanks Christopher!