Handle failing to connect to eslint_d

autoload/ale/fixers/eslint.vim

@@ -25,7 +25,7 @@ endfunction
 function! ale#fixers#eslint#ProcessEslintDOutput(buffer, output) abort
     " If the output is an error message, don't use it.
     for l:line in a:output[:10]
-        if l:line =~# '^Error:'
+        if l:line =~# '\v^Error:|^Could not connect'
             return []
         endif
     endfor

autoload/ale/handlers/eslint.vim

@@ -99,6 +99,13 @@ function! ale#handlers#eslint#Handle(buffer, lines) abort
         \}]
     endif
 
+    if a:lines == ['Could not connect']
+        return [{
+        \   'lnum': 1,
+        \   'text': 'Could not connect to eslint_d. Try updating eslint_d or killing it.',
+        \}]
+    endif
+
     " Matches patterns line the following:
     "
     " /path/to/some-filename.js:47:14: Missing trailing comma. [Warning/comma-dangle]