bitfinex: add DDoSProtection message
bitfinex sent a response like:
```
{"message":"Ratelimit"}
```
presumably in addition to or instead of `ERR_RATE_LIMIT`
Adding this text to error message interpretation
[ci skip]
bitfinex: add DDoSProtection message
bitfinex sent a response like:
```
{"message":"Ratelimit"}
```
presumably in addition to or instead of `ERR_RATE_LIMIT`
Adding this text to error message interpretation
[ci skip]