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`.

test/unit/test_encrypted_blob.py

@@ -19,13 +19,10 @@ def test_decrypt():
     encrypted_data = urandom(64).hex()
     encrypted_blob = EncryptedBlob(encrypted_data)
 
-    # Trying to decrypt random data (in AES_GCM-128) should result in an InvalidTag exception
-    try:
-        encrypted_blob.decrypt(key)
-        assert False, "Able to decrypt random data with random key"
-
-    except InvalidTag:
-        assert True
+    # Trying to decrypt random data (in AES_GCM-128) should result in an InvalidTag exception. Our decrypt function
+    # returns None
+    hex_tx = encrypted_blob.decrypt(key)
+    assert hex_tx is None
 
     # Valid data should run with no InvalidTag and verify
     data = "6097cdf52309b1b2124efeed36bd34f46dc1c25ad23ac86f28380f746254f777"