Add a constructor to SendwithusError that stores content - #78

Description

Hi! I'm a user of this library and I'd like to make a suggestion to improve user experience around error handling.

Motivation and Context

As seen below, the content is being passed in when creating errors and raising them.

On the other hand, none of the error classes doesn't implement their constructor.

From the library users' perspective, this means they have to access the content of the errors like below, which is a bit error-prone (IndexError) and not very readable. It could be obvious to experienced Python developers who have a good understanding of how built-in Exception works internally but I had to spend quite some time to dig into your code to figure this out.

try: 
    api.get_template("INVALID") 
except sendwithus.exceptions.APIError as e: 
    raise MyCustomError(message=e.args[0])                                                                                                                                                                                                                                      

So, my suggestion is to add a constructor to SendwithusError that stores the content value in its content attribute. I think It would be great if the content of the errors could be accessed this way instead.

try: 
    api.get_template("INVALID") 
except sendwithus.exceptions.APIError as e: 
    raise MyCustomError(message=e.content)