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.
