Some cleanups

- Remove unused errors.
- Add doc comments.

src/commands/introducer/add.rs

@@ -1,3 +1,5 @@
+//! introducer add subcommand
+
 use anyhow::Result;
 use sequoia_cert_store::Store;
 use sequoia_openpgp::cert::ValidCert;
@@ -32,7 +34,7 @@ pub async fn dispatch(cmd: IntroducerAddCommand, config: HuskConfigContainer) ->
     if let Some(cert) = certs.first() {
         Introducer::create(&context, cert, cmd.domains)?;
     } else {
-        return Err(CommandError::CertNotFound(fpr).into());
+        return Err(CommandError::CertNotUsable(fpr).into());
     }
 
     Ok(())