fix issue #1637 show error message when preview command fails

[tonymet]

fix #1637

STEPS

SHELL=kaka go run .  --preview 'cat {}'

BEFORE:

[preview window is empty]

AFTER

Preview window:

 exec: "kaka": executable file not found in $PATH

Other Testing

test suite pass

 make test > /dev/null && echo $?                                   
0

Test existing behavior with return code ~= 0

  138   go run .  --preview 'cat {} |grep fzf'
  149   go run .  --preview 'cat {} |grep -v fzf'
  152   go run .  --preview 'cat {} |grep -v fzf'

[junegunn]

Thanks!