Add exception on transaction not found

monero/backends/jsonrpc.py

@@ -295,6 +295,7 @@ _err2exc = {
     -2: exceptions.WrongAddress,
     -4: exceptions.NotEnoughUnlockedMoney,
     -5: exceptions.WrongPaymentId,
+    -8: exceptions.TransactionNotFound,
     -16: exceptions.TransactionNotPossible,
     -20: exceptions.AmountIsZero,
     -32601: MethodNotFound,

monero/exceptions.py

@@ -30,3 +30,5 @@ class TransactionBroadcastError(BackendException):
         self.details = details
         super(TransactionBroadcastError, self).__init__(message)
 
+class TransactionNotFound(AccountException):
+    pass