bitcoind won't start if datadir is specified in the conf file and the user running it has no home dir

tested with debian stretch and bitcoin 0.17.1

```
# sudo -u bitcoin bitcoind -daemon -conf=/usr/local/etc/bitcoin/bitcoin.conf -pid=/run/bitcoind/bitcoind.pid

************************
EXCEPTION: N5boost10filesystem16filesystem_errorE
boost::filesystem::create_directories: Permission denied: "/home/bitcoin"
bitcoin in AppInit()

bitcoind: chainparamsbase.cpp:29: const CBaseChainParams& BaseParams(): Assertion `globalChainBaseParams' failed.
Aborted

# mkhomedir_helper bitcoin

# sudo -u bitcoin bitcoind -daemon -conf=/usr/local/etc/bitcoin/bitcoin.conf -pid=/run/bitcoind/bitcoind.pid
Bitcoin server starting
```


{
    HTTPReply *reply = static_cast<HTTPReply*>(ctx);
    reply->error = err;
}
#endif
/** Class that handles the conversion from a command-line to a JSON-RPC request,
 * as well as converting back to a JSON object that can be shown as result.
 */
class BaseRequestHandler
{
public:
    virtual ~BaseRequestHandler() {}
    virtual UniValue PrepareRequest(const std::string& method, const std::vector<std::string>& args) = 0;
    virtual UniValue ProcessReply(const UniValue &batch_in) = 0;
};
class GetinfoRequestHandler: public BaseRequestHandler