erasure-code: fix erasure_code_benchmark goop (decode)

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. That was done for the encode function but
not the decode function.

http://tracker.ceph.com/issues/9429 Fixes: #9429

Signed-off-by: Loic Dachary <loic-201408@dachary.org>