pgp: fix crash on gpg decrypt fail

The gpg decrypt function was catching errors but not returning them.
This patch returns errors that are caught instead of aerc panicking.

Reported-by: ReK2 <rek2@hispagatos.org>
Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>
Tested-by: ReK2 <rek2@hispagatos.org>
Acked-by: Robin Jarry <robin@jarry.cc>

lib/crypto/gpg/gpgbin/decrypt.go

@@ -26,6 +26,7 @@ func Decrypt(r io.Reader) (*models.MessageDetails, error) {
 			md.Body = bytes.NewReader(orig)
 			return md, nil
 		default:
+			return nil, err
 		}
 	}
 	outRdr := bytes.NewReader(g.stdout.Bytes())