Clearer error message when assets-dir doesn't exist

[sssilver]

Presently, if a Leptos project specifies an assets-dir within [package.metadata.leptos] that doesn't exist on the filesystem, cargo leptos fails with:

       Cargo finished cargo build --package=reel --lib --target-dir=/Users/user/Developer/project/target/front --target=wasm32-unknown-unknown --no-default-features --features=csr
       Front compiling WASM
Error: Mirroring "public" -> "target/site" at `/Users/user/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cargo-leptos-0.2.21/src/compile/assets.rs:136:10`

Caused by:
    No such file or directory (os error 2)

While it's helpful to know that there's an error due to some file or directory not existing, it would be super helpful to know exactly what file or directory the tool is looking for, what that directory's purpose is, and how the user can action the error.