Commit 5b1f9255 (rc: Use the standard `fail` command to report errors,
2019-11-14) replaced uses of "echo -markup {Error}" with "fail".
This made format-buffer do
echo "eval -client $kak_client %{ fail }" | kak -p $kak_session
Unfortunately "fail" fails in the client spawned by "kak -p" and not
in $kak_client where the user would see the message. Correct this.
While at it, clarify the error message, so users immediately know
that the number is the exit code.
Fixes#3254
a0477b1016