server: Verify status code on update check
server: Verify status code on update check

Previously we would try to JSON decode a response body without checking
its status code. This led to log errors like:

```
Error decoding updates info: invalid character 'h' looking for beginning
of value
```

Now we get something like:

```
Failed to check for updates: status: 500 Internal Server Error, body: http.go:96:
failed to fetch updates for UUID c4e50fd6-02cf-4605-8b61-49410c0ace11:
dial tcp 127.0.0.1:26257: socket: too many open files, error: nil
```

glycerine/cockroach,petermattis/cockroach,arjun4084346/cockroach,arjun4084346/cockroach,petermattis/cockroach,paperstreet/cockroach,glycerine/cockroach,sunya123/cockroach,maxlang/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,arjun4084346/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,petermattis/cockroach,maxlang/cockroach,arjun4084346/cockroach,paperstreet/cockroach,paperstreet/cockroach,sunya123/cockroach,vaibhavarora/cockroach,sunya123/cockroach,sunya123/cockroach,vivekmenezes/cockroach,petermattis/cockroach,glycerine/cockroach,arjun4084346/cockroach,arjun4084346/cockroach,maxlang/cockroach,sunya123/cockroach,vivekmenezes/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,glycerine/cockroach,cockroachdb/cockroach,maxlang/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,cockroachdb/cockroach,veteranlu/cockroach,sunya123/cockroach,veteranlu/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,vivekmenezes/cockroach,sunya123/cockroach,sunya123/cockroach,maxlang/cockroach,vivekmenezes/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,glycerine/cockroach,maxlang/cockroach,arjun4084346/cockroach,glycerine/cockroach,paperstreet/cockroach,petermattis/cockroach,vivekmenezes/cockroach,veteranlu/cockroach,petermattis/cockroach,cockroachdb/cockroach,glycerine/cockroach,vivekmenezes/cockroach,veteranlu/cockroach,cockroachdb/cockroach,veteranlu/cockroach,petermattis/cockroach,glycerine/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,petermattis/cockroach,vivekmenezes/cockroach,paperstreet/cockroach,arjun4084346/cockroach,maxlang/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,maxlang/cockroach