[validation] Make script error messages uniform for parallel/single validation

This makes the debug output mostly the same for -par=1 and parallel validation runs. Of course,
parallel validation is non-deterministic in what error it may encounter first if there are
multiple issues. Also, the way certain script-related and non-script-related checks are
performed differs between the two modes still, which may result in discrepancies.

test/functional/feature_nulldummy.py

@@ -57,7 +57,6 @@ class NULLDUMMYTest(BitcoinTestFramework):
         self.extra_args = [[
             f'-testactivationheight=segwit@{COINBASE_MATURITY + 5}',
             '-addresstype=legacy',
-            '-par=1',  # Use only one script thread to get the exact reject reason for testing
         ]]
 
     def create_transaction(self, *, txid, input_details=None, addr, amount, privkey):