[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_csv_activation.py

@@ -99,7 +99,6 @@ class BIP68_112_113Test(BitcoinTestFramework):
         self.noban_tx_relay = True
         self.extra_args = [[
             f'-testactivationheight=csv@{CSV_ACTIVATION_HEIGHT}',
-            '-par=1',  # Use only one script thread to get the exact reject reason for testing
         ]]
         self.supports_cli = False