examples/: Improve error handling on `server-accept'.

* examples/ssshd.scm.in (main): Improve error handling on
  `server-accept': Print exception key on exception, wait 1 second
  before the second try to prevent flooding of the terminal with
  errors.
* examples/echo/server.scm.in (main): Likewise.
* doc/api-servers.texi (Servers): Update description of
  `server-accept'.