erasure-code: fix erasure_code_benchmark goop

Using a stringstream that is only displayed on error when calling the
erasure code factory, instead of cerr. The user expects the output to be
clean when there is no error.

http://tracker.ceph.com/issues/9429 Fixes: #9429

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #2464 from dachary/wip-9429-bench

erasure-code: fix erasure_code_benchmark goop

Reviewed-by: Janne Grunau <j@jannau.net>