feat: return error if verifySignature fail

This prevent hard plumbing and to only use io.Reader interface with
security.
VerifySignature() doesn't return error if verifyKeyRing isn't specified.

Signed-off-by: Guilhem Lettron <guilhem@barpilot.io>

crypto/keyring_streaming.go

@@ -159,6 +159,9 @@ func (msg *PlainMessageReader) Read(b []byte) (n int, err error) {
 	n, err = msg.details.UnverifiedBody.Read(b)
 	if errors.Is(err, io.EOF) {
 		msg.readAll = true
+		if vErr := msg.VerifySignature(); vErr != nil {
+			err = vErr
+		}
 	}
 	return
 }
@@ -174,8 +177,6 @@ func (msg *PlainMessageReader) VerifySignature() (err error) {
 	if msg.verifyKeyRing != nil {
 		processSignatureExpiration(msg.details, msg.verifyTime)
 		err = verifyDetailsSignature(msg.details, msg.verifyKeyRing)
-	} else {
-		err = errors.New("gopenpgp: no verify keyring was provided before decryption")
 	}
 	return
 }