Function load 

pub fn load(
    identity_id: &str,
    origin: &str,
    realm: &str,
    is_proxy: bool,
) -> Result<Option<BrowserCredential>, String>

Look up a stored credential. Ok(None) means “nothing saved for this identity + protection-space” (not an error) — same contract as secret_store::get_optional, which this wraps directly so a transient keychain failure (locked keychain, no Secret Service daemon, permission denied) is distinguishable from “genuinely never saved” at the call site, the same distinction secret_store’s own doc comment argues for.