Create error when you cannot find emcc

[justinmann]

If emcc is not available on the path, the loader fails with a non-intuitive error:
ERROR in ./src/foo.clist
Module build failed: Error: ENOENT: no such file or directory, open '/Users/jusmann/aws/aws-demo/dist/foo.js'
@ ./src/app.ts 7:36-58

Here is the sample app:
https://github.com/justinmann/aws-demo

[mezum]

I fixed it to emit user-friendly message.
Thank you for your report.