rgw: cut short object read if a chunk returns error

Fixes: #8289
Backport: firefly, dumpling
When reading an object, if we hit an error when trying to read one of
the rados objects then we should just stop. Otherwise we're just going
to continue reading the rest of the object, and since it can't be sent
back to the client (as we have a hole in the middle), we end up
accumulating everything in memory.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>