Use catch :default for default error handling in cljs

This is considered normal practice for cljs. See
https://clojure.atlassian.net/browse/CLJS-661 for when it was introduced
and background on it

src/main/frontend/diff.cljs

@@ -68,6 +68,6 @@
 
           :else
           pos))
-      (catch js/Error e
+      (catch :default e
         (log/error :diff/find-position {:error e})
         (count markup)))))