Add an assertion message to the assert-is-empty in LinkedList::~LinkedList, to indicate to users who hit it that it's the fault of the caller, not the fault of MFBT code. No bug, rs=froydnj

mfbt/LinkedList.h

@@ -325,7 +325,12 @@ public:
     : sentinel(mozilla::Move(aOther.sentinel))
   { }
 
-  ~LinkedList() { MOZ_ASSERT(isEmpty()); }
+  ~LinkedList() {
+    MOZ_ASSERT(isEmpty(),
+               "failing this assertion means this LinkedList's creator is "
+               "buggy: it should have removed all this list's elements before "
+               "the list's destruction");
+  }
 
   /*
    * Add aElem to the front of the list.