consistent responses and exceptions

[kemingy]

Currently, starlette will return its own starlette.Response like JSONResponse, while flask and falcon return an inherit instance of pydantic.BaseModel. To reduce users' work, all of these should return the frameworks' own type.

On the other hand, exceptions are not caught by spectree, so it won't verify if it's the correct format as defined.

[kemingy]

flask.abort will return a HTML page instead of JSON body.
falcon.HTTPError has some extra HTTP Status Codes.

[kemingy]

flask response is fixed in #14, falcon response is fixed in #13.