Reported here: https://github.com/bitcoin/bitcoin/pull/10244#issuecomment-378035305

Travis log here: https://travis-ci.org/bitcoin/bitcoin/jobs/361243719#L2810

Note the JSON exception:

```
test_framework.authproxy.JSONRPCException: The mempool was not loaded yet (-1) 
```

That exception was introduced here: https://github.com/bitcoin/bitcoin/commit/cb1e319fe9e198c9c5cf5236fe9af5a3d748b9e8#diff-a0c8f511d90e83aa9b5857e819ced344R1617

cc @jtimon 
{
    if (request.fHelp || request.params.size() != 0) {
        throw std::runtime_error(
            RPCHelpMan{"savemempool",
                "\nDumps the mempool to disk. It will fail until the previous dump is fully loaded.\n",
                {},
                RPCResults{},
                RPCExamples{
                    HelpExampleCli("savemempool", "")
            + HelpExampleRpc("savemempool", "")
                },
            }.ToString());
    }
    if (!g_is_mempool_loaded) {
        throw JSONRPCError(RPC_MISC_ERROR, "The mempool was not loaded yet");
    }
    if (!DumpMempool()) {
        throw JSONRPCError(RPC_MISC_ERROR, "Unable to dump mempool to disk");
    }
    return NullUniValue;
}
bool FindScriptPubKey(std::atomic<int>& scan_progress, const std::atomic<bool>& should_abort, int64_t& count, CCoinsViewCursor* cursor, const std::set<CScript>& needles, std::map<COutPoint, Coin>& out_results) {
    scan_progress = 0;