Make extend_data() error message slightly more informative
Make extend_data() error message slightly more informative

I noticed this because of clippy useless_format error.

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

stratis-storage/stratisd,stratis-storage/stratisd,mulkieran/stratisd,trgill/stratisd,trgill/stratisd,mulkieran/stratisd,stratis-storage/stratisd