clojurescript silently ignores missing externs files possibly leading a developer to chase their tail.
Presently it can be very confusing using advanced compilation if you have made a mistake in the path name of one of your :externs files. This patch makes the compiler print a warning on stderr so you can quickly determine the cause of the broken advanced compilation output.
As a side effect, when doing a basic lein-cljsbuild a warning is always printed:
WARNING: js resource path closure-js/externs does not exist
This is because lein-cljsbuild quietly adds this extra path to your :externs listing without you knowing.