typo'd HTTP method gives misleading error message

Just FYI, I had a typo in my sense query (note the double-G in "GGET")

```
GGET _count
{
"query" : {
"match_all": {}
}
}
```

The error message I got was "Failed to connect to Sense's backend, make sure the Kibana server is running". (Which really confused me, because I'm running Sense within Kibana.)

Reproduce the issue in Sense using the shown GGET _count request and trace how the request error becomes the displayed backend-connection message. Done means a typo in the HTTP method produces an accurate invalid-method error instead of claiming that Kibana's backend is unavailable.

Written by the indexing model from the issue text.