[nvim] Handle SIGHUP in exit handler (#1749)

In recent Nvim versions, an "Error running ..." message is shown even for normal
use cases, such as:

    :Files
    <c-\><c-n>
    :close

Closing the window will :bwipeout! the terminal buffer, because fzf sets
bufhiden=wipe.

When deleting the terminal buffer while fzf is still running, Nvim sends SIGHUP.
This happens for quite some time already, but the bug only manifests since this
commit:

  https://github.com/neovim/neovim/commit/939d9053b

It's The Right Thing to do when the application exited due to a signal.

Before that commit, no "Error running ..." message was shown, because 1 (instead
of 128 + 1 == SIGHUP) was returned which the exit handler in fzf.vim treats as
"NO MATCH".