Return default values for transaction priority instead of throwing an exception

cw_bitcoin/lib/bitcoin_transaction_priority.dart

@@ -22,7 +22,7 @@ class BitcoinTransactionPriority extends TransactionPriority {
       case 2:
         return fast;
       default:
-        throw Exception('Unexpected token: $raw for BitcoinTransactionPriority deserialize');
+        return medium;
     }
   }
 
@@ -73,7 +73,7 @@ class LitecoinTransactionPriority extends BitcoinTransactionPriority {
       case 2:
         return fast;
       default:
-        throw Exception('Unexpected token: $raw for LitecoinTransactionPriority deserialize');
+        return medium;
     }
   }
 

cw_core/lib/monero_transaction_priority.dart

@@ -50,7 +50,7 @@ class MoneroTransactionPriority extends TransactionPriority {
       case 4:
         return fastest;
       default:
-        throw Exception('Unexpected token: $raw for MoneroTransactionPriority deserialize');
+        return automatic;
     }
   }