Give each failure one error value (closes #113)

internal/cli's copies of vault.ErrSecretNotFound, ErrVaultNotFound,
ErrVersionNotFound and ErrSecretExists are removed; the commands wrap the
vault errors. Every error of secret.ReadPassphrase wraps
ErrPassphraseNotRead, so its callers no longer add those words.
ResolveGPGKeyFingerprint returns ErrGPGKeyNotFound for a key the keyring
lacks, recognised by gpg's status line. storeInKeychain returns
errNilDataBuffer. bip85's ErrPasswordTooShort and ErrEncodedTooShort go
with their unreachable checks. Tests that matched these errors' text use
errors.Is.

Model: opus-5-5

internal/cli/crypto.go

@@ -17,7 +17,6 @@ import (
 var (
 	errNotAgeSecretKey = errors.New(
 		"does not contain a valid age secret key")
-	errSecretDoesNotExist = errors.New("does not exist")
 )
 
 // newCryptoCmd builds an encrypt/decrypt command with input/output flags
@@ -245,7 +244,7 @@ func (cli *Instance) Decrypt(secretName, inputFile, outputFile string) error {
 	}
 
 	if !exists {
-		return fmt.Errorf("secret '%s' %w", secretName, errSecretDoesNotExist)
+		return fmt.Errorf("secret '%s' %w", secretName, vault.ErrSecretNotFound)
 	}
 
 	// Get the age secret key from the secret