200 OK returned when max-concurrent-queries limit exceeded

Nobody has claimed this yet.

Does it make more sense if influxdb returns 503?

```
curl -v http://localhost:8086/query?q=show%20databases [18/90225]
* About to connect() to localhost port 8086 (#0)
* Trying 127.0.0.1...
* Connected to localhost (127.0.0.1) port 8086 (#0)
> GET /query?q=show%20databases HTTP/1.1
> User-Agent: curl/7.29.0
> Host: localhost:8086
> Accept: */*
>
< HTTP/1.1 200 OK
< Content-Type: application/json
< Request-Id: c75ec0d4-be90-11e9-acf2-448a5bdc9e68
< X-Influxdb-Build: OSS
< X-Influxdb-Version: 1.7.7rc1
< X-Request-Id: c75ec0d4-be90-11e9-acf2-448a5bdc9e68
< Date: Wed, 14 Aug 2019 12:41:04 GMT
< Transfer-Encoding: chunked
<
{"results":[{"statement_id":0,"error":"max-concurrent-queries limit exceeded(2, 2)"}]}
```

Start by reproducing the request against the /query endpoint while max-concurrent-queries is exceeded, using the response shown in the issue as the baseline. Trace where the limit error is converted into the HTTP response; done means the same condition returns 503 instead of 200 while still reporting the limit error.

Written by the indexing model from the issue text.