Modifies behaviour towards a failed EncryptedBlob decrpytion

The decryption for the `EncryptedBlob` using AES-GCM-128 (the only cipher available atm) raises an `InvalidTag` exception. This was not properly captured by the watcher making it crash. This behavior was already discovered during the `EncryptedBlob` unit testing and left to be fixed in the `Watcher` unit testing.

However, making the EncryptedBlob raise such an exception may not be a good practice, since other ciphers may run into different exceptions. Therefore, the `EncryptedBlob` has been modified to return None upon facing a decryption issue, the `BlockProcessor` will detect that and return a None justice_txm and justice_txid. Upon receiving a None `justice_txid` the `Watcher` will delete the appointment without notifiying the `Responder`.

pisa/block_processor.py

@@ -74,15 +74,17 @@ class BlockProcessor:
                     # ToDo: #20-test-tx-decrypting-edge-cases
                     justice_rawtx = appointments[uuid].encrypted_blob.decrypt(dispute_txid)
                     justice_txid = bitcoin_cli.decoderawtransaction(justice_rawtx).get('txid')
-                    matches.append((locator, uuid, dispute_txid, justice_txid, justice_rawtx))
-
                     logger.info("Match found for locator.", locator=locator, uuid=uuid, justice_txid=justice_txid)
 
                 except JSONRPCException as e:
                     # Tx decode failed returns error code -22, maybe we should be more strict here. Leaving it simple
                     # for the POC
+                    justice_txid = None
+                    justice_rawtx = None
                     logger.error("Can't build transaction from decoded data.", error=e.error)
 
+                matches.append((locator, uuid, dispute_txid, justice_txid, justice_rawtx))
+
         return matches
 
     # DISCUSS: This method comes from the Responder and seems like it could go back there.