CLEANUP: stconn: always use se_fl_set_error() to set the pending error

CLEANUP: stconn: always use se_fl_set_error() to set the pending error

In mux-h2 and mux-quic we still had two places manually setting
SE_FL_ERR_PENDING or SE_FL_ERROR depending on the EOS state, instead
of using se_fl_set_error() which takes care of the condition. Better
use the specialized function for this, it will allow to centralize
the conditions. Note that this will be needed to fix a bug.