Make STRICTENC invalid pubkeys fail the script rather than the opcode.

This turns STRICTENC turn into a softforking-safe change (even though it
is not intended as a consensus rule), and as a result guarantee that using
it for mempool validation only results in consensus-valid transactions in
the mempool.

src/script/script_error.cpp

@@ -61,6 +61,8 @@ const char* ScriptErrorString(const ScriptError serror)
             return "Dummy CHECKMULTISIG argument must be zero";
         case SCRIPT_ERR_DISCOURAGE_UPGRADABLE_NOPS:
             return "NOPx reserved for soft-fork upgrades";
+        case SCRIPT_ERR_PUBKEYTYPE:
+            return "Public key is neither compressed or uncompressed";
         case SCRIPT_ERR_UNKNOWN_ERROR:
         case SCRIPT_ERR_ERROR_COUNT:
         default: break;