Throw when trying to remove a child highlighter that does not exists

src/highlighter_group.cc

@@ -39,6 +39,8 @@ void HighlighterGroup::add_child(HighlighterAndId&& hl)
 
 void HighlighterGroup::remove_child(StringView id)
 {
+    if (not m_highlighters.contains(id))
+        throw runtime_error{format("no such child: {}", id)};
     m_highlighters.remove(id);
 }