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

pkg/bip85/bip85.go

@@ -59,16 +59,6 @@ var (
 	// ErrInvalidBase85PwdLen is returned when the Base85 password length
 	// is out of range.
 	ErrInvalidBase85PwdLen = errors.New("pwdLen must be between 10 and 80")
-	// ErrPasswordTooShort is returned when the derived material is
-	// shorter than the requested password length. It carries only the
-	// middle of the message, which the caller composes as
-	// "derived password length <n> is shorter than requested length <m>",
-	// so the emitted text is unchanged.
-	ErrPasswordTooShort = errors.New("is shorter than requested length")
-	// ErrEncodedTooShort is returned when the encoded material is shorter
-	// than the requested password length. Composed as
-	// "encoded length <n> is less than requested length <m>".
-	ErrEncodedTooShort = errors.New("is less than requested length")
 )
 
 // Version bytes for extended keys
@@ -381,14 +371,8 @@ func DeriveBase64Password(
 	// Remove any padding
 	encodedStr = strings.TrimRight(encodedStr, "=")
 
-	// Slice to the desired password length
-	if len(encodedStr) < int(pwdLen) {
-		return "", fmt.Errorf(
-			"derived password length %d %w %d",
-			len(encodedStr), ErrPasswordTooShort, pwdLen,
-		)
-	}
-
+	// Slice to the desired password length: 64 bytes of entropy leave 86
+	// characters, the most pwdLen allows
 	return encodedStr[:pwdLen], nil
 }
 
@@ -411,14 +395,8 @@ func DeriveBase85Password(
 	// Base85 encode all 64 bytes of entropy using the RFC1924 character set
 	encoded := encodeBase85WithRFC1924Charset(entropy)
 
-	// Slice to the desired password length
-	if len(encoded) < int(pwdLen) {
-		return "", fmt.Errorf(
-			"encoded length %d %w %d",
-			len(encoded), ErrEncodedTooShort, pwdLen,
-		)
-	}
-
+	// Slice to the desired password length: 64 bytes of entropy give 80
+	// characters, the most pwdLen allows
 	return encoded[:pwdLen], nil
 }