Add useful error message for plugin load

If a plugin has previously been installed, but the path is no longer
valid, `rake setup` will fail with an unexpected error due to the file
not existing.

Instead, we want to present the user with what the issue is and how to
resolve the problem.

Author: sarahsehr

bundler/lib/bundler/plugin.rb

@@ -342,7 +342,30 @@ def load_plugin(name)
       # done to avoid conflicts
       path = index.plugin_path(name)
 
-      Gem.add_to_load_path(*index.load_paths(name))
+      paths = index.load_paths(name)
+
+      all_paths_valid = true
+      paths.each do |p|
+        next if File.directory?(p)
+
+        message = <<~MESSAGE
+          Failed loading plugin #{name}:
+
+          Plugin path #{p} does not exist. This can happen if the plugin was \
+          installed with a different version of Ruby that has since been uninstalled.
+
+          If you would like to reinstall the plugin, run:
+
+          bundler plugin uninstall #{name} && bundler plugin install #{name}
+
+          Continuing without installing plugin #{name}.
+        MESSAGE
+        Bundler.ui.warn message
+        all_paths_valid = false
+      end
+      return unless all_paths_valid
+
+      Gem.add_to_load_path(*paths)
 
       load path.join(PLUGIN_FILE_NAME)
 

bundler/spec/bundler/plugin_spec.rb

@@ -333,5 +333,27 @@
         end.to output("win\n").to_stdout
       end
     end
+
+    context "the plugin load_path is invalid" do
+      before do
+        allow(index).to receive(:load_paths).with("foo-plugin").and_return(["invalid-file-name"])
+      end
+
+      it "outputs a useful warning" do
+        msg =
+          "Failed loading plugin foo-plugin:\n\n" \
+          "Plugin path invalid-file-name does not exist. This can happen if the plugin was " \
+          "installed with a different version of Ruby that has since been uninstalled.\n\n" \
+          "If you would like to reinstall the plugin, run:\n\n" \
+          "bundler plugin uninstall foo-plugin && bundler plugin install foo-plugin\n\n" \
+          "Continuing without installing plugin foo-plugin.\n"
+
+        expect(Bundler.ui).to receive(:warn).with(msg)
+
+        Plugin.hook(Bundler::Plugin::Events::EVENT1)
+
+        expect(subject.loaded?("foo-plugin")).to be_falsey
+      end
+    end
   end
 end