Feature(sendError): add unwrap interface

[sonalys]

Just a brief description of my problem:

I'm trying to validate if a FROM address is valid, and I'm looking into the SMTP Error codes.
I noticed that you receive the error codes, and hide then behind the SendError struct.

It would be helpful for me to have a direct access to these error codes through textproto.Error, instead of parsing them from the error message.

We Have the errors.As function, that tries to go error to error from the error stack, until the type assertion Matches.
I'm implementing the Unwrap interface on SendError for that reason, so we can do errors.As and extract any internal error from the SendError struct.

Another possible solution would be if you also stored the ErrorCode on the SendError struct.

[wneessen]

Thanks for the PR @sonalys. I am fine with adding the Unwrap() on SendError but if your main concern is the error code, I agree that adding the error code into the senderror struct would be the better solution. I'll create an issue for us to add this.