What does Jonas describe as a "terrible punishment, an overwhelming statement of failure" in The Giver? Who is he referring to?

Jonas is talking about release, which is a terrible punishment and a failure for the community.  He is referring to the jet pilot who flew over the community by mistake.

Jonas’s community controls its citizens’ every move.  The goal of Sameness is to ensure that all people are safe and no one is ever uncomfortable.  For example, when Jonas sees a jet plane fly overhead, he is afraid.  Apparently everyone in the community is, because the Speaker orders everyone inside.  It turns out that the jet flying over the community was just an accident, but the damage is done.

NEEDLESS TO SAY, HE WILL BE RELEASED, the voice had said, followed by silence. There was an ironic tone to that final message, as if the Speaker found it amusing; and Jonas had smiled a little, though he knew what a grim statement it had been. (Ch. 1)

For a “contributing citizen” to be released is a big deal.  Old people are released all of the time.  Apparently they can no longer contribute.  However, a person who breaks three rules, or makes a major mistake, can also be released.  This is considered a failure of the community.

Most of the citizens have no idea what release actually is.  All they know is the euphemism and the idea that once a person is released, he or she goes to Elsewhere.  As far as they know, Elsewhere is a place. However, when they release a person what they are really doing is killing him with a lethal injection.

There is another instance of release other than punishment and the elderly.  If a baby does not meet the requirements of the community, they terminate it.  Jonas witnesses the release of a twin, killed for being the smaller one of an identical set.

As he continued to watch, the newchild, no longer crying, moved his arms and legs in a jerking motion. Then he went limp. The head fell to the side, his eyes half open. Then he was still. (Ch. 19)

Release of a newchild is considered sad, because babies have done nothing wrong.  Release of the elderly is celebrated, because they have lived a long life.  Release of adults who have broken the rules is considered a failure of the community to get people to conform.