Add more context to Sprockets::FileNotFound error messages

[carols10cents]

Sort of related to #251. If the error messages I had gotten had included more information, perhaps it wouldn't matter that the documentation hadn't helped me.

This is with a brand new rails 4.2.5.1 app and sprockets 3.5.2.

The relevant part of that issue to this issue is that I had a file in app/assets/other_javascripts/f.js (a sibling directory to app/assets/javascripts, not a subdirectory) and I tried to require it in app/assets/javascripts/application.js by adding require ../other_javascripts/f. () When I run rake assets:precompile in this state, I get this error message:

Sprockets::FileNotFound: couldn't find file '../other_javascripts/f' under '/Users/carolnichols/Ruby/sprockets-test-app/app/assets/javascripts' with type 'application/javascript'

Now, I know app/assets/javascripts isn't the only place sprockets is looking, since the comment at the top of application.js says I can also put js in lib/assets/javascripts and vendor/assets/javascripts and reference them directly here.

I also happen to know that if I instead use require f, sprockets DOES find app/assets/other_javascripts/f.js (https://github.com/carols10cents/sprockets-test-app/commit/cb1315e645e5f2118776caf62b6667f7cbe323b5), so it's clearly looking more places.

It'd be great if the error message for Sprockets::FileNotFound showed me all the paths sprockets was looking for assets, and showed me how it was resolving the paths, so that I could understand and correct the path I'm putting into application.js.

[5minpause]

Hi @schneems I created a pull request, that displays the load paths together with the regular error message. Perhaps this is helpful? Any feedback is appreciated.
I opened the PR to the branch 3.x. Does this make sense? Or is development only done in master?

[schneems]

Ugh, I only merged that behavior into 3.x and not into master. There is a branch I need to get green on tests first #322.

[schneems]

This was fixed in #322