The previous commit made it over-zealous

The previous change helped to understand the acutal issue:
EPIPE because our wrapped program did not handle EAGAIN,
see https://github.com/libsndfile/libsndfile/pull/820

This commit reverts 51c0e396
in the sense to not fall back (because the EPIPE would then
be seen after the write()), but improves error reporting.