Add error trace to Functions with xpcall

[Tazmondo]

Not sure what your stance on this is, but I personally think it's good for DX so I figured I'd make a PR and you can decide whether to merge or close.
There's no pressure at all to respond quickly - I know you're preoccupied with thinking about zap so feel free to come back to this later.
The code isn't particularly long so there's nothing lost if it isn't merged.

For reference, evaera's promise library uses xpcall for its stack traces so it's a battle tested method for this sort of thing.

[Tazmondo]

Error appearance:

With corresponding callback body:

ErrorFunction:SetCallback(function(Player)
	assert(1 == 2, "Assert failed.") -- Line 36
	return ""
end)

[Tazmondo]

Summarizing the conversation on discord regarding this PR:

People may rely on the pattern of a function silently erroring on the server and returning that error with its value to the client.
But this change would cause a warning to be emitted every time the function errors with a string value (even when intentional).
So it's not necessarily a breaking change but it's not a simple improvement to merge in.

Perhaps we could add this as an option when creating Functions, similar to how you can set whether an event is reliable or unreliable? Like SilenceErrors: boolean? or something.