feat(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/.changelog.d/1442.changed

@@ -0,0 +1 @@
+Raise `ValueError` when the txid for an input is not present in `prev_txes` during `btc.sign_tx`

python/src/trezorlib/btc.py

@@ -334,6 +334,10 @@ def sign_tx(
 
         # 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