z_try_recv should return z_result with possible errors E_NODATA and E_CLOSED

[milyin]

Currently z_try_recv functions for channels returns bool which is false when channel is closed. No data situation is handled by testing received data for gravestone state (with z_check)

After discussion with @DenisBiryukov91 and @sashacmc it was decided that no data situation should be another kind of error. This allows to not call z_check on received data.

This removes the only place where z_check is absolutely necessary. After this change we are free to make z_check rework for #492