Don't do any formatting of outputs on the server side.

* server/scm/lazycatd.scm (lazycat-exec, lazycat-diff): Send
  results to a client as a list instead of squashing all in a
  string.  Add comments.
* ui/lc (lazycat-exec, lazycat-diff): Improve.
* ui/cli/pretty-format.scm (format-output, format-output-list)
  (format-diff): New functions.
* server/scmapi.c (scm_send_msg): Throw an exception on error.
  (lazycat_throw): New function.
* server/scm/host.scm (host-send-message): Catch an exception.
Fix two bugs in handling "diff --abort" command.

* ui/lc: Fix a bug in handling "diff --abort" command.
* server/scm/lazycatd.scm: Likewise.