Implement --identity command

src/error.rs

@@ -1,9 +1,19 @@
 use std::fmt;
 use std::io;
+use yubikey_piv::{key::RetiredSlotId, Serial};
+
+use crate::USABLE_SLOTS;
 
 pub enum Error {
+    InvalidSlot(u8),
     Io(io::Error),
     MultipleCommands,
+    MultipleIdentities,
+    MultipleYubiKeys,
+    NoIdentities,
+    NoMatchingSerial(Serial),
+    SlotHasNoIdentity(RetiredSlotId),
+    TimedOut,
     YubiKey(yubikey_piv::Error),
 }
 
@@ -24,11 +34,38 @@ impl From<yubikey_piv::error::Error> for Error {
 impl fmt::Debug for Error {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
+            Error::InvalidSlot(slot) => writeln!(
+                f,
+                "Invalid slot '{}' (expected number between 1 and 20).",
+                slot
+            )?,
             Error::Io(e) => writeln!(f, "Failed to set up YubiKey: {}", e)?,
             Error::MultipleCommands => writeln!(
                 f,
                 "Only one of --generate, --identity, --list, --list-all can be specified."
             )?,
+            Error::MultipleIdentities => writeln!(
+                f,
+                "This YubiKey has multiple age identities. Use --slot to select a single identity."
+            )?,
+            Error::MultipleYubiKeys => writeln!(
+                f,
+                "Multiple YubiKeys are plugged in. Use --serial to select a single YubiKey."
+            )?,
+            Error::NoIdentities => {
+                writeln!(f, "This YubiKey does not contain any age identities.")?
+            }
+            Error::NoMatchingSerial(serial) => {
+                writeln!(f, "Could not find YubiKey with serial {}.", serial)?
+            }
+            Error::SlotHasNoIdentity(slot) => writeln!(
+                f,
+                "Slot {} does not contain an age identity or compatible key.",
+                USABLE_SLOTS.iter().position(|s| s == slot).unwrap() + 1
+            )?,
+            Error::TimedOut => {
+                writeln!(f, "Timed out while waiting for a YubiKey to be inserted.")?
+            }
             Error::YubiKey(e) => match e {
                 yubikey_piv::error::Error::NotFound => {
                     writeln!(f, "Please insert the YubiKey you want to set up")?