Add error string for CLEANSTACK script violation, preventing an "unknown error" if the CLEANSTACK error condition is set.

src/script/script_error.cpp

@@ -73,6 +73,8 @@ const char* ScriptErrorString(const ScriptError serror)
             return "Witness version reserved for soft-fork upgrades";
         case SCRIPT_ERR_PUBKEYTYPE:
             return "Public key is neither compressed or uncompressed";
+        case SCRIPT_ERR_CLEANSTACK:
+            return "Extra items left on stack after execution";
         case SCRIPT_ERR_WITNESS_PROGRAM_WRONG_LENGTH:
             return "Witness program has incorrect length";
         case SCRIPT_ERR_WITNESS_PROGRAM_WITNESS_EMPTY: