function joinStacks (newStack, originalStack) {
newStack = popStack(newStack);

if (newStack && originalStack) {
  return newStack + '\n\n' + originalStack;
}
else {
  return newStack || originalStack;
}
}
Appends the original {@link Error#stack} property to the new Error's stack.

@param {string} newStack
@param {string} originalStack
@returns {string}