Throw error for invalid manifest format

[djfarrelly]

We should return an error here:

buffer-static-upload/main.go

Lines 195 to 211 in e7db242

	// FormatManifest returns the file version manifest in json or csv format
	func FormatManifest(fileVersions map[string]string, format string) ([]byte, error) {
	if format == "json" {
	return json.MarshalIndent(fileVersions, "", " ")
	}
	if format == "csv" {
	b := &bytes.Buffer{}
	wr := csv.NewWriter(b)
	for filename, uri := range fileVersions {
	row := []string{filename, uri}
	wr.Write(row)
	}
	wr.Flush()
	return b.Bytes(), nil
	}
	return nil, nil
	}