This fixes two bugs:

First, FS Logs API endpoint only propagated error back to user if it was
encoded with code, which isn't common.  Other errors get suppressed and
callers get an empty response with 200 error code.  Now, these endpoints
return  a 500 status code along with the error message.

Before
```
$ curl -v "http://127.0.0.1:4646/v1/client/fs/logs/qwerqwera?follow=false&offset=0&origin=start&region=global&task=redis&type=stdout"; echo
*   Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to 127.0.0.1 (127.0.0.1) port 4646 (#0)
> GET /v1/client/fs/logs/qwerqwera?follow=false&offset=0&origin=start&region=global&task=redis&type=stdout HTTP/1.1
> Host: 127.0.0.1:4646
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 200 OK
< Vary: Accept-Encoding
< Vary: Origin
< Date: Fri, 04 Oct 2019 19:47:21 GMT
< Content-Length: 0
<
* Connection #0 to host 127.0.0.1 left intact
```

After
```
$ curl -v "http://127.0.0.1:4646/v1/client/fs/logs/qwerqwera?follow=false&offset=0&origin=start&region=global&task=redis&type=stdout"; echo
*   Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to 127.0.0.1 (127.0.0.1) port 4646 (#0)
> GET /v1/client/fs/logs/qwerqwera?follow=false&offset=0&origin=start&region=global&task=redis&type=stdout HTTP/1.1
> Host: 127.0.0.1:4646
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 500 Internal Server Error
< Vary: Accept-Encoding
< Vary: Origin
< Date: Fri, 04 Oct 2019 19:48:12 GMT
< Content-Length: 60
< Content-Type: text/plain; charset=utf-8
<
* Connection #0 to host 127.0.0.1 left intact
alloc lookup failed: index error: UUID must be 36 characters
```

Second, we return 400 status code for request validation errors.

Before
```
$ curl -v "http://127.0.0.1:4646/v1/client/fs/logs/qwerqwera"; echo
*   Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to 127.0.0.1 (127.0.0.1) port 4646 (#0)
> GET /v1/client/fs/logs/qwerqwera HTTP/1.1
> Host: 127.0.0.1:4646
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 500 Internal Server Error
< Vary: Accept-Encoding
< Vary: Origin
< Date: Fri, 04 Oct 2019 19:47:29 GMT
< Content-Length: 22
< Content-Type: text/plain; charset=utf-8
<
* Connection #0 to host 127.0.0.1 left intact
must provide task name
```

After
```
$ curl -v "http://127.0.0.1:4646/v1/client/fs/logs/qwerqwera"; echo
*   Trying 127.0.0.1...
* TCP_NODELAY set
* Connected to 127.0.0.1 (127.0.0.1) port 4646 (#0)
> GET /v1/client/fs/logs/qwerqwera HTTP/1.1
> Host: 127.0.0.1:4646
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 400 Bad Request
< Vary: Accept-Encoding
< Vary: Origin
< Date: Fri, 04 Oct 2019 19:49:18 GMT
< Content-Length: 22
< Content-Type: text/plain; charset=utf-8
<
* Connection #0 to host 127.0.0.1 left intact
must provide task name
```