rpcserver: attempt to fix uncaught exception.

Addresses #5560.

```asio::ip::address::to_string()``` can throw in some cases, which I believe winxp may be hitting.

I've simulated the case locally by adding:
```
            if (bindAddress.is_v6())
            {
                boost::system::error_code ec(boost::asio::error::invalid_argument);
                boost::asio::detail::throw_error(ec);
            }
```
after the to_string() in order to force the error.

Without the change, bitcoind dies with:
```
EXCEPTION: N5boost16exception_detail10clone_implINS0_19error_info_injectorINS_6system12system_errorEEEEE       
Invalid argument       
bitcoin in AppInit()
```
With the change, it continues and logs:
```
ERROR: Binding RPC on address ::1 port 36522 failed: Invalid argument
```

Even if this doesn't fix the crash as intended, I believe it's safe and more correct anyway.