QUIC: reset qc->error to zero again.

QUIC: reset qc->error to zero again.

Following the previous change that removed posting a close event
in OpenSSL compat layer, now ngx_quic_close_connection() is always
called on error path with either NGX_ERROR or qc->error set.

This allows to remove a special value -1 served as a missing error,
which simplifies the code.  Partially reverts d3fb12d77.

Also, this improves handling of the draining connection state, which
consists of posting a close event with NGX_OK and no qc->error set,
where it was previously converted to NGX_QUIC_ERR_INTERNAL_ERROR.
Notably, this is rather a cosmetic fix, because drained connections
do not send any packets including CONNECTION_CLOSE, and qc->error
is not otherwise used.