Improve output if no_lookup_host_duplicates fails
Improve output if no_lookup_host_duplicates fails

If the test fails, output the offending addresses and a helpful error message.
Also slightly improve legibility of the preceding line that puts the addresses
into a HashMap.

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust