Make plugin connection much more robust, with better errors

plugin/src/Reconciler.lua

@@ -124,6 +124,13 @@ function Reconciler:_reify(item)
 	return rbx
 end
 
+--[[
+	Clears any state that the Reconciler has, effectively restarting it.
+]]
+function Reconciler:clear()
+	self._routeMap:clear()
+end
+
 --[[
 	Apply the changes represented by the given item to a Roblox object that's a
 	child of the given instance.