Fix HTTPException message

[imptype]

1. Fix HTTPException erroring when getting content types other than application/json.
   

2. Be more explicit with the response data. Sometimes it doesn't have the "code" key in it, but it is a valid json.
   

[jnsougata]

As we are passing the resp object ref inside the Exception class so it will be good to hide the content_type check within the class for clarity, rather than putting it in the global _handler.

[imptype]

As we are passing the resp object ref inside the Exception class so it will be good to hide the content_type check within the class for clarity, rather than putting it in the global _handler.

We can't move the content_type check inside the class because you can't use await in __init__. There are other ways to async init but it's messier than what we have now.

[jnsougata]

As we are passing the resp object ref inside the Exception class so it will be good to hide the content_type check within the class for clarity, rather than putting it in the global _handler.

We can't move the content_type check inside the class because you can't use await in __init__. There are other ways to async init but it's messier than what we have now.

In that case, we can just add those awaited values to the error obj before raising exception or we can add response attr to the Error class.

Error.response.status_code if we need to check status code.
await error.response.read() if we need the body for exception handling and we can add some async method like to_string()

[imptype]

If we REALLY want to avoid checking the response's content-type outside and put it inside the HTTPException class instead for the sake of making the code in https.py look less and cleaner, the only decent way to do it is using an async class method

So instead of

        if resp.status >= 400:
            raise HTTPException(resp, await resp.json())

it's now

        if resp.status >= 400:
            raise await HTTPException.create(resp)

and the check and async reads are in the async create method.