Panic when bucket doesn't exists

[nitisht]

Server logs panic error when a bucket as the Parseable storage is not provided.

thread 'actix-rt|system:0|arbiter:2' panicked at 'called `Result::unwrap()` on an `Err` value: Storage(NoSuchBucket(NoSuchBucket { message: Some("The specified bucket does not exist") }))', server/src/handlers/logstream.rs:79:60

We need to handle the properly and log a better error - and not start the server if bucket doesn't exist.