Julia: Better error messages for code loading

[KristofferC]

The current error message:

ERROR: ArgumentError: Module dfsohgfdg not found in current path.
Run `Pkg.add("dfsohgfdg")` to install the dfsohgfdg package.

is not very good. current path doesn't make sense, Pkg.add might not be the solution, it doesn't distinguish between the "identify"- and `"location" phase etc.

We should probably thread something through code loading that keeps track of where we looked for stuff so that we can give a good error message.

@StefanKarpinski

[KristofferC]

This is getting very important to get done because people rightly getting very confused. I am away for a couple of days and cannot work on it during those days. I have an extreme WIP branch here https://github.com/JuliaLang/julia/tree/kc/loading_debug_error which tries to thread some "backtrace" through code loading and record stuff to be able to replay what happened and why code loading failed. Not sure how well thought out it is...

[polytomous]

I'm eagerly awaiting some more progress on this item. (It's very relevant to my interests)
I tossed two small comments on the latest commit here: https://github.com/JuliaLang/julia/commit/8e34ee9768d07b5e1122555ba408933a8ba68aaa