When Closure exits with non-zero code but no stderr, output is not minified and there are no errors

[ian-craig]

I hit this when the plugin decided to use 'native' Closure but vcredist was missing on a build agent. The .exe was returning a non-zero exit code but no stderr output, and the result was that Webpack reported success but the JS was not minified.

It looks like the compilerProcess error handling logic in the plugin just misses this edge case.

I'll submit a PR shortly.