Wrap errors in SMTPError to comply with Adapter.deliver/2 spec

[adamu]

In #177, this adapter was updated to comply with the new spec in Bamboo v2.

However, as suggested in this comment, it is still necessary to wrap the errors in SMTPError. This is because the Adapter.error/0 spec requires that the error must be a Exception.t() | String.t().

Currently, bamboo_smtp is returning the underlying error tuple from gen_smtp, which is a violation of the spec and causes Elixir to complain when Bamboo raises the exception. An example I have seen in server logs:

ArgumentError: raise/1 and reraise/2 expect a module name, string or exception as the first argument, got: {:network_failure, '74.125.204.108', {:error, :closed}}

This PR wraps the errors in SMTPError to comply with the spec, so that it can be successfully raised by Bamboo without causing ArgumentErrors from raise.