go-client: changed statements' Err type to string, tagged by error

[farshidtz]

The HTTP API json response in case of errors has "error": "string" format. The Go client was instead looking for err key and an error typed value.

Sample error responses from InfluxDB v0.9.5.1:

{
  "error": "error parsing query: found EOF, expected identifier at line 1, char 35"
}

{
  "results": [
    {
      "error": "retention policy duration must be at least 1h0m0s"
    }
  ]
}

[otoolep]

Thanks @farshidtz -- I see you have signed the CLA.

+1

@sparrc @corylanou

[sparrc]

+1 LGTM

[otoolep]

Thanks @farshidtz

[farshidtz]

@otoolep It's the least I could do.