`_add`: Panic when adding a component that does not exist in registry.

[max-wells]

Problem

❯ cargo run --bin ui add button demo_button demo_buttomkzmkrzmelkr
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.36s
     Running `target/debug/ui add button demo_button demo_buttomkzmkrzmelkr`

thread 'main' panicked at src/command_add/dependencies.rs:23:68:
Failed to resolve all dependencies: "Target component 'demo_buttomkzmkrzmelkr' not found in index"
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Behavior expected

We should inform the user with a message like this :
"🔸 Component not found in registry"

And still let the others (valid) components be added to the project.