Show in loclist if linter created output but parser couldn't parse

After reading a lot of issues from the project (for example https://github.com/dense-analysis/ale/issues/3401) and creating my own in https://github.com/dense-analysis/ale/issues/4184, one comment from @w0rp caught my eye:

> The regular expression for parsing the flake8 errors will need to be updated. It's probably tripping up on the ECE606: in the filename, and needs to be a bit less greedy.

I was thinking if it's feasible to add a message to loclist when the linter outputs something but the parser couldn't parse. This way we know that a parser update is needed and avoid searching for bugs in vim, or in the linter itself. Something along the lines (just a suggestion):

```
returned output:
<>
But nothing was parsed. Check if it's an environment problem or if it's the case to open an issue to update the parser.
```
or

``` returned output but nothing was parsed. Check :ALEInfo to see if it's an environment problem or if it's the case to open an issue to update the parser.```

The important thing IMHO is to create a pattern (use the loclist to warn or just ALEInfo?) and keep it (since other situations like https://github.com/dense-analysis/ale/issues/4155 would be logged in a consistent manner).