exportCtrl mapped every export failure to 500 Internal Server Error, so
requesting a backup for a non-existent site (e.g. wrong -s/--site) came
back as a misleading 500 instead of a client error — inconsistent with
the rest of the admin/public API, which returns 400 + ErrSiteNotFound
for site-lookup failures.
Add an engine.ErrSiteNotFound sentinel (wrapped at the bolt db-lookup so
the existing "site %q not found" message is unchanged) and map it to 400
+ rest.ErrSiteNotFound in exportCtrl; genuine internal failures (gzip
close/write) still return 500.
Dmitry VerkhoturovandUmputun
2544d80f98