validation: only check input scripts once

Previously, we would check failing input scripts twice when considering
a transaction for the mempool, in order to distinguish policy failures
from consensus failures. This allowed us both to provide a different
error message and to discourage peers for consensus failures. Because we
are no longer discouraging peers for consensus failures during tx relay,
and because checking a script can be expensive, only do this once.

Also renames non-mandatory-script-verify-flag error to
mempool-script-verify-flag-failed.

test/functional/rpc_packages.py

@@ -123,8 +123,8 @@ class RPCPackagesTest(BitcoinTestFramework):
         assert_equal(testres_bad_sig, self.independent_txns_testres + [{
             "txid": tx_bad_sig_txid,
             "wtxid": tx_bad_sig_wtxid, "allowed": False,
-            "reject-reason": "mandatory-script-verify-flag-failed (Operation not valid with the current stack size)",
-            "reject-details": "mandatory-script-verify-flag-failed (Operation not valid with the current stack size), " +
+            "reject-reason": "mempool-script-verify-flag-failed (Operation not valid with the current stack size)",
+            "reject-details": "mempool-script-verify-flag-failed (Operation not valid with the current stack size), " +
                               f"input 0 of {tx_bad_sig_txid} (wtxid {tx_bad_sig_wtxid}), spending {coin['txid']}:{coin['vout']}"
         }])