erasure-code: return error using `ss` not derr

erasure-code: return error using `ss` not derr

log is not very visible from user's perspective. we'd better return the
error message using the input parameter, if the technique is
unsupported.

also, remove the trailing newline, as `ss` will be sent back to user,
it's the the front-end's responsibility to format the error message.

Signed-off-by: Kefu Chai <kchai@redhat.com>