foo.vim

let s:buf = []

function! s:error(msgs)
  redraw
  echohl ErrorMsg
  for line in a:msgs
    echomsg line
  endfor
  echohl NONE
endfunction

function! s:on_stdout(id, data)
  let s:buf += a:data
endfunction

function! s:on_exit(id)
  call s:error(filter(s:buf, '!empty(v:val)'))
endfunction

call jobstart('echo foo && echo bar', {
      \ 'on_stdout':  function('s:on_stdout'),
      \ 'on_exit':    function('s:on_exit'),
      \ })