If no exception occurs

local function throw_if_true(param)
  local ok, result = pcall(function()
    if param then
      error("test exception", 0)
    end
    return "done"
  end)
  if ok then
    -- No error: result is the returned value
    print("else: " .. result)
  else
    print("except")
  end
end

throw_if_true(true)
-- except
throw_if_true(false)
-- else: done