change get api deal with the 500 code

[ghost]

If you cat a wrong hash, the ipfs http api will response code 500
and with message. In this case if we use the response boy and cover it
to stream or byte ,it will not the true content of the hash code,it will
be the content of the 'Wrong Message'.

So we need check the response code by {response.isSuccessful} to insure
we go the right response.

OKHTTP: <-- 500 Internal Server Error http://127.0.0.1:5001/api/v0/cat/QmZC6gALejeRt2TAHHMnEkcEXeyX3XUSZ4yRkpt43UkM5A?timeout=30s (30002ms)
OKHTTP: Access-Control-Allow-Headers: X-Stream-Output, X-Chunked-Output, X-Content-Length
OKHTTP: Access-Control-Expose-Headers: X-Stream-Output, X-Chunked-Output, X-Content-Length
OKHTTP: Content-Type: application/json
OKHTTP: Server: go-ipfs/0.4.18-190117
OKHTTP: Trailer: X-Stream-Error
OKHTTP: Vary: Origin
OKHTTP: Date: Tue, 26 Feb 2019 03:18:31 GMT
OKHTTP: Transfer-Encoding: chunked
OKHTTP: {"Message":"failed to get block for QmZC6gALejeRt2TAHHMnEkcEXeyX3XUSZ4yRkpt43UkM5A: context deadline exceeded","Code":0,"Type":"error"}
OKHTTP: <-- END HTTP (136-byte body)