Respond with 404 in ActiveStorage::DiskController#show when file missing

`ActiveStorage::DiskController#show` generates a 404 Not Found response when
the requested file is missing from the disk service. It previously raised
`Errno::ENOENT`.

activestorage/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   `ActiveStorage::DiskController#show` generates a 404 Not Found response when
+    the requested file is missing from the disk service. It previously raised
+    `Errno::ENOENT`.
+
+    *Cameron Bothner*
+
 *   `ActiveStorage::Blob#download` and `ActiveStorage::Blob#open` raise
     `ActiveStorage::FileNotFoundError` when the corresponding file is missing
     from the storage service. Services translate service-specific missing object

activestorage/app/controllers/active_storage/disk_controller.rb

@@ -13,6 +13,8 @@ def show
     else
       head :not_found
     end
+  rescue Errno::ENOENT
+    head :not_found
   end
 
   def update

activestorage/test/controllers/disk_controller_test.rb

@@ -31,6 +31,14 @@ class ActiveStorage::DiskControllerTest < ActionDispatch::IntegrationTest
     assert_equal " worl", response.body
   end
 
+  test "showing blob that does not exist" do
+    blob = create_blob
+    blob.delete
+
+    get blob.service_url
+    assert_response :not_found
+  end
+
 
   test "directly uploading blob with integrity" do
     data = "Something else entirely!"