A wrong passphrase gives an error that says so (closes #157)

A passphrase that did not decrypt the passphrase unlocker failed with age's
"no identity matched any of the recipients" and advice to give the vault a
new unlocker from its mnemonic, which would replace an unlocker that works.
GetIdentity now turns that age error into "wrong passphrase for unlocker
<id>", wrapping the new secret.ErrWrongPassphrase, and withMnemonicAdvice
leaves it alone, as it does when no passphrase was read. A passphrase from
SB_UNLOCK_PASSPHRASE and a typed one both reach this code. A priv.age that
is not an age file keeps the advice.

Judgement call: age cannot tell a wrong passphrase from a damaged key inside
a priv.age whose header reads, so that case now reads as a wrong passphrase.

Model: opus-5-5

internal/secret/passphraseunlocker.go

@@ -16,6 +16,10 @@ import (
 // empty. A passphrase unlocker that fails with it was not tried.
 var ErrPassphraseNotRead = errors.New("failed to read passphrase")
 
+// ErrWrongPassphrase is wrapped in the error of GetIdentity when the
+// passphrase does not decrypt the unlocker's private key.
+var ErrWrongPassphrase = errors.New("wrong passphrase")
+
 // PassphraseUnlocker represents a passphrase-protected unlocker
 type PassphraseUnlocker struct {
 	Directory  string
@@ -73,6 +77,12 @@ func (p *PassphraseUnlocker) GetIdentity() (*age.X25519Identity, error) {
 		Debug("Failed to decrypt unlocker private key",
 			"error", err, "unlocker_id", p.GetID())
 
+		// age says that no identity matched when the passphrase is wrong
+		var noMatch *age.NoIdentityMatchError
+		if errors.As(err, &noMatch) {
+			return nil, fmt.Errorf("%w for unlocker %s", ErrWrongPassphrase, p.GetID())
+		}
+
 		return nil, fmt.Errorf("failed to decrypt unlocker private key: %w", err)
 	}
 	defer privKeyBuffer.Destroy()