Previously calling add_transaction with a malformed Transaction obj could
result in an exception late in the flow, after the walletdb was already side-effected.
Rollback of such side-effects is not implemented :/
but this small patch should at least cover and prevent some common cases.
```
File "/opt/electrum/electrum/address_synchronizer.py", line 358, in add_transaction
self.db.add_transaction(tx_hash, tx)
File "/opt/electrum/electrum/json_db.py", line 42, in wrapper
return func(self, *args, **kwargs)
File "/opt/electrum/electrum/wallet_db.py", line 1434, in add_transaction
tx = tx_from_any(str(tx))
File "/opt/electrum/electrum/transaction.py", line 1339, in tx_from_any
raise SerializationError(f"Failed to recognise tx encoding, or to parse transaction. "
```
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