Propagate exceptions from parallel where possible

[SquidDev]

In the original implementation of our prettier runtime errors (#1320), we wrapped the errors thrown within parallel functions into an exception object. This means the call-stack is available to the catching-code, and so is able to report a pretty exception message.

Unfortunately, this was a breaking change, and so we had to roll that back. Some people were pcalling the parallel function, and matching on the result of the error. For example:

local ok, result = pcall(parallel.waitForAny, f, g)
if not ok and result == "Terminated" then
  -- ...
else
  -- ...
end

This is a second attempt at this, using a technique I've affectionately dubbed "magic throws". The parallel API is now aware of whether it is being pcalled or not, and thus able to decide whether to wrap the error into an exception or not:

I think there are some risks with this approach. It's not obvious to users whether you'll get the fancy error reporting or not, and that could be confusing — the "magic" in "magic throws" is not a necessarily a good thing!

I still think this change is worth merging (we can always revert it if it proves to be a mistake), but definitely open to feedback.

I'd quite like to extend this to approach to other bits of the code — we could do something similar to provide better error messages for require.