s3 retrieve can fail with non-obvious error message if MetadataFileExtension is not provided

[discentem]

func TestS3StoreRetrieve(t *testing.T) {
	mTime, _ := time.Parse("2006-01-02T15:04:05.000Z", "2014-11-12T11:45:26.371Z")
	// create bucket content
	bucketfs, err := testutils.MemMapFsWith(map[string]testutils.MapFile{
		"someObject": {
			Content: []byte("tla"),
			ModTime: &mTime,
		},
	})
	assert.NoError(t, err)

	fakeS3Server := aferoS3Server{
		buckets: map[string]afero.Fs{
			// create a bucket in our fake s3 server with the content
			"aFakeBucket": *bucketfs,
		},
	}

	localFs, err := testutils.MemMapFsWith(map[string]testutils.MapFile{
		"someObject.cfile": {
			Content: []byte(`{
				"name": "someObject",
				"checksum": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
				"date_modified": "2014-11-12T11:45:26.371Z"
			   }`),
			ModTime: &mTime,
		},
	})

	store := S3Store{
		Options: Options{
			BackendAddress:        "s3://aFakeBucket",
			MetadataFileExtension: "cfile",
		},
		fsys:         *localFs,
		awsRegion:    "us-east-1",
		s3Uploader:   fakeS3Server,
		s3Downloader: fakeS3Server,
	}

	err = store.Retrieve(context.Background(), "someObject") # NO .cfile EXTENSION HERE
	assert.NoError(t, err)

}

This results in the following error:

json marshal failed: invalid character 'l' in literal true (expecting 'r')

We should catch the user if they don't provide whatever MetadataFileExtension is in their input

cavorite retrieve thing.cfile <--- good input
cavorite retrieve thing <--- bad input, should fail early