Regression in error responses from /api/v0/files/rm

When one causes an error, eg. by attempting to remove an MFS file that does not exist, the error response returned by go-ipfs 0.11.0 is not a valid JSON.

## Problem (0.11.0)

Error response in 0.11.0 is plain text:

```console
> curl -X POST -vvv 'http://127.0.0.1:5001/api/v0/files/rm?arg=/foo/keks' ~
* Trying 127.0.0.1:5001...
* Connected to 127.0.0.1 (127.0.0.1) port 5001 (#0)
> POST /api/v0/files/rm?arg=/foo/keks HTTP/1.1
> Host: 127.0.0.1:5001
> User-Agent: curl/7.79.1
> Accept: */*
>
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Access-Control-Allow-Headers: X-Stream-Output, X-Chunked-Output, X-Content-Length
< Access-Control-Expose-Headers: X-Stream-Output, X-Chunked-Output, X-Content-Length
< Content-Type: application/json
< Server: go-ipfs/0.11.0
< Trailer: X-Stream-Error
< Vary: Origin
< X-Chunked-Output: 1
< Date: Tue, 14 Dec 2021 01:12:02 GMT
< Transfer-Encoding: chunked
<
"/foo/keks: parent lookup: file does not exist"
* Connection #0 to host 127.0.0.1 left intact
```

## Before (0.10.0)

0.10.0 had correct error response (JSON):

```console
> curl -X POST -vvv 'http://127.0.0.1:5001/api/v0/files/rm?arg=/foo/keks' ~
* Trying 127.0.0.1:5001...
* Connected to 127.0.0.1 (127.0.0.1) port 5001 (#0)
> POST /api/v0/files/rm?arg=/foo/keks HTTP/1.1
> Host: 127.0.0.1:5001
> User-Agent: curl/7.79.1
> Accept: */*
>
* Mark bundle as not supporting multiuse
< HTTP/1.1 500 Internal Server Error
< Access-Control-Allow-Headers: X-Stream-Output, X-Chunked-Output, X-Content-Length
< Access-Control-Expose-Headers: X-Stream-Output, X-Chunked-Output, X-Content-Length
< Content-Type: application/json
< Server: go-ipfs/0.10.0
< Trailer: X-Stream-Error
< Vary: Origin
< Date: Tue, 14 Dec 2021 01:07:30 GMT
< Transfer-Encoding: chunked
<
{"Message":"parent lookup: file does not exist","Code":0,"Type":"error"}
* Connection #0 to host 127.0.0.1 left intact
```