The `ErrAlreadyExists` error is used for 304 statuses, which
is not an error-condition, so should probably not be defined
as part of the errdefs package.
This patch removes the `ErrAlreadyExists` interface, and related
helpers, as it was currently not used.
Note that a 304 status can fulfil certain use-cases, but (refering
to https://www.codetinkerer.com/2015/12/04/choosing-an-http-status-code.html)
could probably be handled by a 200 OK, unless we want to perform
caching in the client.
If we do want to use 304 statuses, perhaps we need a separate class
of "errors" for this (?).
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
