fix: clean error when no sites after site-filter

[Teufelchen1]

Hi! 👋

When accidentally running website-stalker run all, I got this error:

thread 'main' panicked at 'Site filter filtered everything out.', src/main.rs:130:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

which I disliked.

This PR changes that, the error now looks like this:

Error: The Site-filter filtered everything out.
There are no sites tracked for your query.
Tipp: Run 'website-stalker run --all' to stalk all tracked sites.

I believe this is better because:

• We can do a graceful exit here, no danger, no debugging needed, no panic
• Shows clearly this is an error - as opposed to failure/fault
• Shows what the user might did wrong
• Shows a possible solution for this error

Have a nice day!