remote: dispatch: Remove return value from make_nonnull_* helpers

remote: dispatch: Remove return value from make_nonnull_* helpers

After conversion to g_strdup, the helpers now always return success.
Remove the return value to simplify the callers.

Note that many occurrences of these is in the code generated by
gendispatch.pl. Since gendispatch aggregates many cases together an
incremental conversion would require more invasive changes to
gendispatch for the time of conversion which doesn't make sense.

Also in many cases the helper was the last place where the 'error:'
label was used and thus also those conversions must be included in this
patch.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
ACKed-by: Eric Blake <eblake@redhat.com>