rgw: return error if accessing object in non-existent bucket

Fixes: #7064
Instead of trying to access the object, which is impossible as we don't
even have a proper bucket info. Up until now we ended up creating an
empty pool and eventually returning ENOENT, this fix catches the issue
earlier in the process.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit 3ed68eb9fac9b3d0bf9111867d609f9ea08fb202)