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/unlockers_add_test.go

@@ -5,6 +5,7 @@ import (
 	"path/filepath"
 	"testing"
 
+	"git.eeqj.de/sneak/secret/internal/secret"
 	"git.eeqj.de/sneak/secret/internal/vault"
 	"github.com/awnumar/memguard"
 	"github.com/spf13/afero"
@@ -98,7 +99,7 @@ func TestAddPGPUnlockerUnknownKey(t *testing.T) {
 
 	err := instance.addPGPUnlocker(cmd)
 
-	require.ErrorContains(t, err, "failed to resolve GPG key fingerprint")
+	require.ErrorIs(t, err, secret.ErrGPGKeyNotFound)
 	assertDirEntries(t, base,
 		filepath.Join(testVaultDir(listTestVaultName), listTestUnlockersDirName),
 		listTestUnlockerDirOne)