fade: Improve error diagnostics.

fade: Improve error diagnostics.

This adds a new generic error code "invalid mixer channel", which is
returned from both ALSA and OSS in case the specified mixer channel
could not be set.

Prior to this patch, ALSA would return a bad channel error also when
the given value was out of range. This is fixed by replacing the now
unused ALSA_MIX_BAD_ELEM error code by the new ALSA_MIX_RANGE code.

Finally, para_fade is changed to no longer print the list of available
channels after it failed to set the initial mode.