add message error instead utf8 panic error when running the update command

[Johjo]

Hello,

i try to use gpm and i fail on the command gpm update.

I get this error :

λ gpm update
 INFO  gpm::gpm::command::update > running the "update" command
Updating all repositories
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Custom { kind: InvalidData, error: "stream did not contain valid UTF-8" }', src\gpm\command\update.rs:39:42

I see in update.rs source that the file causing the error is source.list. I convert it to utf 8 and everything was OK.

Maybe can you add an error message instead letting rust panic ?

Thanks for your job and sorry for my english ;)

Jonathan