Fix 2891 - eslint not showing errors.

ESLint errors are contained in an array that can contain different
stuff other than JSON error messages. This patch iterates over the whole
array ignoring any non-json data.

autoload/ale/handlers/eslint.vim

@@ -84,11 +84,14 @@ function! s:CheckForBadConfig(buffer, lines) abort
 endfunction
 
 function! s:parseJSON(buffer, lines) abort
-    try
-        let l:parsed = json_decode(a:lines[-1])
-    catch
-        return []
-    endtry
+    let l:parsed = []
+
+    for l:line in a:lines
+        try
+            let l:parsed = extend(l:parsed, json_decode(l:line))
+        catch
+        endtry
+    endfor
 
     if type(l:parsed) != v:t_list || empty(l:parsed)
         return []