Return after handling an error when getting a file

[babolivier]

Fixes #55

It looks like the code assumes reject returns after handling an error, which doesn't seem to be the case in javascript.
Also improve the error handling a bit so the client doesn't get Unhandled server error.

Introduced in #47

[squahtx]

LGTM!

I wonder if this is something a linter would catch. Setting up eslint would be a good investment if we're going to be maintaining this project long term.

[babolivier]

Setting up eslint would be a good investment if we're going to be maintaining this project long term.

That's a good point.
I don't hope we're going to maintain this long term, since I'd like there to be a Python replacement sooner than later, but even then setting up a linter doesn't sound like something that would be super non-trivial, and it sounds quite valuable, I'll look into it.