python: Raise ValueError for non-existant tx hashes

If the device asks for a tx_hash which is not present in prev_txes,
raise a ValueError with some more detailed messaging about the missing
hash rather than the default dictionary lookup failure of KeyError.

python/src/trezorlib/btc.py

@@ -279,6 +279,8 @@ def copy_tx_meta(tx: messages.TransactionType) -> messages.TransactionType:
 
         # Device asked for one more information, let's process it.
         if res.details.tx_hash is not None:
+            if res.details.tx_hash not in prev_txes:
+                raise ValueError(f"Previous transaction {res.details.tx_hash.hex()} not available")
             current_tx = prev_txes[res.details.tx_hash]
         else:
             current_tx = this_tx