Checking external requires throws before a working require is found

https://github.com/kmalakoff/gulp-module-system/blob/master/module_systems/local-shim.coffee#L99

If window.require can't find the dependency it'll throw and prevent the locally injected external require from actually being tried.

Open local-shim.coffee around line 99 and trace how window.require handles a missing dependency before the injected external require is reached. Confirm that a failed lookup does not stop the local external require from being tried, and verify that a working external dependency is resolved successfully.

Written by the indexing model from the issue text.