Replace 400 code with 200 and empty array for response without comments

[akellbl4]

As I can see if a user doesn't have any comments then the server returns error 400 with data:

{
    "code": 1,
    "details": "can't get comment by user id",
    "error": "no comments for user twitter_500c776ee5c34f78a7e410277bf9f8f1ecd34145 in store"
}

Could we change this error to status 200 and empty array?

[umputun]

i have changed the behavior to return 200 in this case. however, it also required to allow "empty" responses. Prior to the change the response was

	resp := struct {
		Comments []store.Comment `json:"comments,omitempty"`
		Count    int             `json:"count,omitempty"`
	}{}

in order to return {comments:[], count:0} I had to drop omitempty. Pls, check this pr branch as this is API change and the only thing we may affect here is UI

[akellbl4]

@umputun works fine!