recode_with_coeff should detect incompatible shards

In fec module, when a set of shards are not compatible, they can still be recoded together with coeffs, and result in an actual shard (see current doc-test). Suggested fix:

• rename as recode_with_coeffs_unchecked
• write recode_with_coeffs as a wrapper returning a Result<..., KomodoError>, that makes the same checks as recode_random plus checking coeffs vector length compatibility