Feedback on failure to lint

With ale's default configuration it enables every known linter and opportunistically attempts to run them all, silently failing in most cases. While this yolo approach might be a reasonable out of the box, a more trusted ale experience can be obtained by explicitly configuring working linters only on known filetypes.

When using such a tightly controlled ale configuration, one might want to get immediate visual feedback whenever any linter fail to execute. Preferably a linter-like error on the first line of the buffer. Such functionality must be hidden behind a configuration option defaulting to disabled, of course. I made an attempt at , but gave up and settled with the following work-around in my vimrc instead:

```vim
function! ALELinterProblem()
let l:history = ale#history#Get(bufnr(''))
if match(l:history, "'status': 1")
echom l:history
sign define missing_linter text=!! texthl=Error
exe ':sign place 1 line=1 name=missing_linter'
endif
endfunction

augroup ALEMissingLinter
autocmd User ALELintPost unsilent call ALELinterProblem()
augroup END
```

While the triggering on the ALELintPost event kind of addresses the problem, having support in ale for reporting linter failure in a noticeable manner would be preferred.

Does the ALE team agree this functionality could at all be desired? If so, would anyone be able to give any hints on how to add it more completely and cleanly than on my branch linked above?