AVERROR_NUMEXPECTED is used only in the image muxer and demuxer, and
has a too much specific meaning, which is better explained through a
log message. Thus it can be replaced by AVERROR(EINVAL).

This breaks API.
Signed-off-by: Stefano Sabatini <stefano.sabatini-lala@poste.it>
Signed-off-by: Anton Khirnov <anton@khirnov.net>