internal/http_api: fixed error messages quoting

[FilWisher]

Error messages that get bubbled up are returned from the API wrapped
using a call to fmt.Sprintf({"message":"%s"}, data). Double quotes
appearing inside data are unescaped causing incorrectly formatted JSON
(which breaks the UI).

[FilWisher]

The API request to /api/topics failed but the JSON response is also incorrectly formatted causing a UI error. The offending error message is:

{"message":"UPSTREAM_ERROR: Failed to query any nsqlookupd: Get http://127.0.0.1:4160/topics: net/http: HTTP/1.x transport connection broken: malformed HTTP response "\x00\x00\x00\x0eE_BAD_PROTOCOL""}

You can see the double quotes around \x00\x00..._BAD_PROTOCOL aren't escaped correctly.

[jehiah]

Thanks for catching this issue!

[mreiferson]

I wonder if this explains #727 ???

[ploxiln]

Thanks for the update. There's just one tiny bit which go is not happy with:

internal/http_api/api_response.go:8:2: imported and not used: "strconv"