src: Catch “kill exceptions”’cleanly

When the `kill` command is called in the `-E` CLI flag, the resulting
exception is not caught and crashes the server.

This commit allows the server to terminate cleanly.

Since `KakEnd` hooks also need to be executed should the user run a
command like `kak -E 'kill 0'`, the execution of `KakBegin` hooks is
now performed *before* the command provided in `-E` is executed. The
documentation for `KakBegin` (executed after the `-E` command prior to
this commit) consequently becomes more truthful, as it states:

	KakBegin session name
		kakoune has started, this hook is called just after
		reading the user configuration files

Fixes #3972

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