Improve session termination diagnostic

[christianparpart]

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When PTY gets closed, print close-reason (exit code / signal name)

The user should still be able to use shortcuts or vi-like normal mode when that happend. But a special input binding would then close the session (maybe defaulting to ESC?)

Action name for input bindings could be DestroySession as a generally applicable action to destroy sessions, which is particularily interesting for future work, where we want to support more than one session (tabs, splits)