detect using Distributed and warn about incompatibility

[carlocab]

This is related to #300.

The README says

Cells can contain arbitrary Julia code, and you can use external libraries.

This is a little misleading, since you're not supposed to use Distributed or Threads, unless I've done something wrong in the notebook I was working on.

It would be nice if this were stated in some form in the documentation, and for error messages related to using these features in a Pluto notebook to be a bit more informative. At the moment attempting to use Distributed leads to errors about missing workspaces, while Threads fails in the browser silently, and only registers errors in the terminal you started Pluto from.

[fonsp]

Ive added it to the wiki

[fonsp]

@carlocab Can you post some errors that you usually get? We can write a pattern for those and generate a custom error message. For writing the pattern: the more errors you post the better.

[carlocab]

Sure. As a start, there are the ones documented here.

I have a few more different variations of a workspace not defined error. I'll try and reproduce them this afternoon.