rgw: generalize error handling in RGWShardCollectCR

rgw: generalize error handling in RGWShardCollectCR

RGWShardCollectCR was hard-coded to ignore ENOENT errors and print a
'failed to fetch log status' error message. this moves that logic into a
handle_result() virtual function. it also exposes the member variables
'status' and 'max_concurrent' as protected, so they can be consulted or
modified by overrides of handle_result() and spawn_next()

Signed-off-by: Casey Bodley <cbodley@redhat.com>