Give Error messages not just error codes

[LibPQ.jl does this really well](https://github.com/invenia/LibPQ.jl/blob/master/src/error_codes.jl)

Here is [the docs with all the codes](https://curl.haxx.se/libcurl/c/libcurl-errors.html), but there might be another list of coddes somewhere that is easier to regex into julia code

Right now we get things passed on to FTPClient.jl which result in unuseful messages like:
```
FTPClientError("Failed to upload /foo.bar", 0x0000003c).
```

But it could have been:
```
CURLE_PEER_FAILED_VERIFICATION (60)
The remote server's SSL certificate or SSH md5 fingerprint was deemed not OK.
```