refactor: remove process::exit from library code, return Result instead

[duna-feline]

Changes

• Remove all 5 std::process::exit(1) calls from src/config.rs
• RLLVMConfig::new() and load_path() now return Result<Self, Error>
• New try_default() fallible method extracted from Default::default()
• Added ConfigError(String) variant to Error enum
• Panic boundary pushed to rllvm_config() OnceLock init (appropriate for global singleton)
• Binary files unchanged — process::exit is fine there

Why

Library code should never call process::exit() — it prevents callers from handling errors gracefully. This is especially important for users who may want to use rllvm as a library.