server-func.c (server-accept): Throw `guile-ssh-error' on error.

* ssh/server-func.c (server-accept): Throw `guile-ssh-error' on
  error.
* examples/ssshd.scm (main): Handle `guile-ssh-error' on
  `server-accept'.
* examples/echo/server.scm (main): Likewise.
* doc/api-servers.texi (Servers): Update `server-accept'
  documentation.  Add an example.
* NEWS: Update.