Merge bitcoin/bitcoin#30278: test: cover more errors for `signrawtransactionwithkey` RPC

e2779ce98b test: cover more errors for `signrawtransactionwithkey` RPC (brunoerg)

Pull request description:

  This PR adds test coverage for the following errors for the `signrawtransactionwithkey` RPC:

  - Invalid private key
  - TX decode failed

  For reference: https://maflcko.github.io/b-c-cov/total.coverage/src/rpc/rawtransaction.cpp.gcov.html

ACKs for top commit:
  maflcko:
    ACK e2779ce98b
  kevkevinpal:
    ACK [e2779ce](e2779ce98b)
  tdb3:
    ACK e2779ce98b
  BrandonOdiwuor:
    Code Review ACK e2779ce98b

Tree-SHA512: 41c7e990684b60645cf4ccec8aad5ebbe61da221871eb3c1685b2bb1eebda58b29358502cb1525b7c7a2b612e2bebf449ed0bae14ab663b4641c528a9c013b5b

test/functional/rpc_signrawtransactionwithkey.py

@@ -126,10 +126,20 @@ class SignRawTransactionWithKeyTest(BitcoinTestFramework):
         privkeys = [self.nodes[0].get_deterministic_priv_key().key]
         assert_raises_rpc_error(-8, "'all' is not a valid sighash parameter.", self.nodes[0].signrawtransactionwithkey, tx, privkeys, sighashtype="all")
 
+    def invalid_private_key_and_tx(self):
+        self.log.info("Test signing transaction with an invalid private key")
+        tx = self.nodes[0].createrawtransaction(INPUTS, OUTPUTS)
+        privkeys = ["123"]
+        assert_raises_rpc_error(-5, "Invalid private key", self.nodes[0].signrawtransactionwithkey, tx, privkeys)
+        self.log.info("Test signing transaction with an invalid tx hex")
+        privkeys = [self.nodes[0].get_deterministic_priv_key().key]
+        assert_raises_rpc_error(-22, "TX decode failed. Make sure the tx has at least one input.", self.nodes[0].signrawtransactionwithkey, tx + "00", privkeys)
+
     def run_test(self):
         self.successful_signing_test()
         self.witness_script_test()
         self.invalid_sighashtype_test()
+        self.invalid_private_key_and_tx()
 
 
 if __name__ == '__main__':