(Bug 4196) Rename DW::EmailPost::Reply -> DW::EmailPost::Comment

Consistency in naming is nice.
(Bug 4196) Return a tuple instead of a "magic status message"
(Bug 4196) Tweak error message from "auth" to "secret address"

Matches how we refer to it elsewhere
(Bug 4196) Assorted minor tweaks for style/clarity
(Bug 4196) Rename auth to parse_destination

We don't actually authenticate, just looking for a potential
user/journal this should be posted to.
(Bug 4196) Add method "should_handle"
(Bug 4196) err sub instead of $self->{error}  ... and return pattern

Also renames $self->error to $self->send_error, to make it easier to
distinguish between setting the message ($self->err) and sending out the
error notification ($self->send_error)