Error object returned by pcall() declared to always be a string

[Feuermurmel]

Description

pcall() is declared to return LuaMultiReturn<[true, R] | [false, string]>, but in Lua, the error object can be anything. This leads to problem when the error object is used in a context where it matters whether it can be something other than a string.

To reproduce

Consider the following example (playground):

function foo(this: void) {
 throw {}
}

let result = pcall(foo)

if (!result[0]) {
 print("error: " + result[1])
}

Function foo() throws an empty object. To allow concatenation with a string, a call to tostring() needs to be inserted, but it isn't. So the concatenation in the argument to print() fails:

Lua execution error:
	[string "--[[ Generated with https://github.com/TypeSc..."]:7: attempt to concatenate a table value (field '?')

If the correct type is added to the declaration of result, the call to tostring() is inserted:

let result: [true, void] | [false, unknown] = pcall(foo)

Generated code:

--[[ Generated with https://github.com/TypeScriptToLua/TypeScriptToLua ]]
function foo()
 error({}, 0)
end
result = {pcall(foo)}
if not result[1] then
 print("error: " .. tostring(result[2]))
end

Expected behavior

I think that the return type of pcall() should be changed to LuaMultiReturn<[true, R] | [false, unknown]>. The same applies to coroutine.resume().