improve traceback of import errors

[valq7711]

No description provided.

[atsepkov]

Not related to the PR, but what's the state of ES6 imports (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import) and what are their limitations, if any, compared to RS? The import mechanism here predates the ES6 version, and has been great, but does it make sense to eventually drop ES5 support and replace this with the modern version?

[valq7711]

It's impossible to create and run module in browser (you can compile but to run you have to send it to the server and get it back).
It will force rs-users to dive into webpack if it's necessary to bundle modules into one (or few) file(s).
If we drop the existing import implementation, Node.js and webpack will become hard dependencies - I think, it's bad news for pythonic-people.
On the other hand, module-in-browser could be produced by setting private_scope to false and adding JS('export {...}')/JS('import {...} from ...') even now! - as for me, this is quite enough.