Bugfix: test/functional/rpc_psbt: Remove check for specific error message that depends on uncertain assumptions

When converttopsbt is called with a signed transaction, it either fails with "TX decode failed" if one or more inputs were segwit, or "Inputs must not have scriptSigs and scriptWitnesses" otherwise.
Since no effort is made by the test to ensure the inputs are segwit or not, avoid checking the exact message used.
The error code is still checked to ensure it is of the correct kind of failure.

test/functional/rpc_psbt.py

@@ -144,9 +144,10 @@ class PSBTTest(BitcoinTestFramework):
         self.nodes[0].decodepsbt(new_psbt)
 
         # Make sure that a psbt with signatures cannot be converted
+        # Error could be either "TX decode failed" (segwit inputs causes parsing to fail) or "Inputs must not have scriptSigs and scriptWitnesses"
         signedtx = self.nodes[0].signrawtransactionwithwallet(rawtx['hex'])
-        assert_raises_rpc_error(-22, "TX decode failed", self.nodes[0].converttopsbt, signedtx['hex'])
-        assert_raises_rpc_error(-22, "TX decode failed", self.nodes[0].converttopsbt, signedtx['hex'], False)
+        assert_raises_rpc_error(-22, "", self.nodes[0].converttopsbt, signedtx['hex'])
+        assert_raises_rpc_error(-22, "", self.nodes[0].converttopsbt, signedtx['hex'], False)
         # Unless we allow it to convert and strip signatures
         self.nodes[0].converttopsbt(signedtx['hex'], True)