Raise HTTPError in send_request function so client can better handle the error

[shifucun]

Now the send_request function always throws ValueError [1] which is incorrect. For Http error, it should pass it along to client so client can identify if it's server error, argument error or data error.

[1] https://github.com/datacommonsorg/api-python/blob/master/datacommons/utils.py#L113

[manojbalaji1]

@shifucun can you give an example instance, like for what input, its actual output/error, expected output/error?