Improve "key not found" error messages for string/symbol keys

We don't know how to represent arbitrary objects as strings at this
low level, but this takes care of two very common cases.

M2/Macaulay2/d/hashtables.dd

@@ -340,7 +340,12 @@ export lookup1force(object:HashTable,key:Expr,keyhash:hash_t):Expr := (
 	  bucket = bucket.next;
 	  );
      if lockit then unlock(object.mutex);
-     buildErrorPacket("key not found in hash table"));
+     msg := "key not found in hash table";
+     when key
+     is x:stringCell do msg = msg + ": \"" + x.v + "\""
+     is x:SymbolClosure do msg = msg + ": " + x.symbol.word.name
+     else nothing;
+     buildErrorPacket(msg));
 export lookup1force(object:HashTable,key:Expr):Expr := lookup1force(object,key,hash(key));
 export lookup1Q(object:HashTable,key:Expr,keyhash:hash_t):bool := (
      lockit := object.Mutable && !object.beingInitialized;