Since the send error indicate an error during the mail delivery, in my opinion it should be reset when a re-send is initiated, so that the senderror field always represents the latest delivery error. The send error should be checked after mail delivery and before a retry is started.
This PR introduces the `SendError` type which implements the error interface.
A new `senderror` field has been added to the `Msg` as well, so introduce this type to it.
I've also added different error variables that indicate the different things that can go wrong during mail delivery. These variables can be checked for, for each `Msg` using the `errors.As` method
The `Error()` method of `SendError` will return a detailed error string on why the `Msg` could not be delivered.
Additionally, `HasSendError()` and `SendError()` methods have been added to `Msg`. While `HasSendError()` simply returns a bool in case a `Msg` failed during delivery, the `SendError()` will return the full `SendError` error interface.
`Client.Send()` provides the possibility to send multiple `*Msg` in one go. If one of the `*Msg` caused an error with the sending mail server, we were returning completely, while not processing any `*Msg` that came after the failing message.
This PR fixes this behaviour by processing each message first and then return a accumulated error in case any of the `*Msg` processing failed
Additionally, this PR separates the `Client.Send()` method into two different versions. One that makes use of the new `errors.Join()` functionality that is introduced with Go 1.20 and one that handles it the old way for any supported version lower than Go 1.20
