More useful error message if index.html is missing when running `lein

ring server`  #2195

src/metabase/routes.clj

@@ -10,7 +10,9 @@
 
 (defn- index [_]
   (-> (if (@(resolve 'metabase.core/initialized?))
-        (stencil/render-string (slurp (io/resource "frontend_client/index.html"))
+        (stencil/render-string (slurp (or (io/resource "frontend_client/index.html")
+                                          ;; Occasionally people try to launch from 'lein ring server' and move stuff around. In that case give them a useful error message.
+                                          (throw (Exception. "Cannot find './resources/frontend_client/index.html'."))))
                                {:bootstrap_json (json/generate-string (setting/public-settings))})
         (slurp (io/resource "frontend_client/init.html")))
       resp/response