Break out proxmox-router's HttpError into it's own crate so that it can
be used without pulling in proxmox-router.
This commit also implements `Serialize` for `HttpError` so that it can
be returned from perlmod bindings, allowing Perl code to access the
status code as well as the message.
Also add some smoke-tests to make sure that the `http_bail` and
`http_err` macros actually produce valid code.
Suggested-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>
