cert: fix dropped errors (#961)

cert/cert.go

@@ -272,6 +272,9 @@ func EncryptAndMarshalSigningPrivateKey(curve Curve, b []byte, passphrase []byte
 		},
 		Ciphertext: ciphertext,
 	})
+	if err != nil {
+		return nil, err
+	}
 
 	switch curve {
 	case Curve_CURVE25519:

cert/crypto.go

@@ -77,6 +77,9 @@ func aes256Decrypt(passphrase []byte, kdfParams *Argon2Parameters, data []byte)
 	}
 
 	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return nil, err
+	}
 
 	nonce, ciphertext, err := splitNonceCiphertext(data, gcm.NonceSize())
 	if err != nil {