Occasional E474: Trailing characters in ale additions

I occasionally get this error while editing text (seems to be correlated to slow merlin responses, but hard to reproduce):

```
calling function 97_NeoVimCallback[29]..94_HandleExit[44]..36_callback(1, ['{"class":"return","value":[],"notifi...der":2,"ppx":18,"typer":1,"error":2}}'])

line 1: let l:errors = []
line 2: let l:json = join(a:lines, "\n")
line 3: let l:resp = json_decode(l:json)
Exception thrown: Vim(let):E474: Trailing characters: {"class":"return","value":[],"notifications":[],"timing":{"total":22,"query":1,"pp":0,"reader":2,"ppx":18,"typer":1,"error":2}}

function 97_NeoVimCallback[29]..94_HandleExit[44]..36_callback aborted
```

I have commented out the ALE stuff for now. The outlining command works great.