Some cleanups

- Remove unused errors.
- Add doc comments.

src/commands/introducer/remove.rs

@@ -1,3 +1,5 @@
+//! introducer remove subcommand
+
 use anyhow::Result;
 
 use sequoia_openpgp::{Fingerprint, KeyHandle};
@@ -35,7 +37,7 @@ pub async fn dispatch(cmd: IntroducerRemoveCommand, config: HuskConfigContainer)
     if let Some(cert) = certs.first() {
         Introducer::remove(&context, cert, cmd.demote)?;
     } else {
-        return Err(CommandError::CertNotFound(fpr).into());
+        return Err(CommandError::CertNotUsable(fpr).into());
     }