Add helper method to raise an exception for error status codes

[Kentzo]

Long story short

aiohttp should provide a helper's method to check for errors in responses. Something like request's raise_for_error.

[Kentzo]

Something like:

ClientResponse:
    …
    def raise_for_status(self):
         http_error_msg = ''

        if 400 <= self.status< 500:
            http_error_msg = 'Client Error: {}'.format(self.reason)
        elif 500 <= self.status < 600:
            http_error_msg = 'Server Error: {}'.format(self.reason)

        if http_error_msg:
            raise aiohttp.HttpProcessingError(code=self.status, message=http_error_msg)

(borrowed from requests)

[asvetlov]

Good to have.
Not sure about exact exception type though.

[lock]

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