print a warning when :externs file paths can't be found.

Description

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.

Environment

Linux 64bit

java version "1.7.0_65"
OpenJDK Runtime Environment (IcedTea 2.5.3) (7u71-2.5.3-0ubuntu0.14.04.1)
OpenJDK 64-Bit Server VM (build 24.65-b04, mixed mode)

Assignee

David Nolen

Reporter

import

Approval

None

Patch

Code

Fix versions

Affects versions

Priority

Minor
Configure