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

[msdos]

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):

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

or

<linter> 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 <linter> 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).

[hsanson]

The parser works by matching a pattern so there is no way to know if nothing was returned because the parser failed or because there were no errors reported by the linter.

[msdos]

Usually when something wrong happens with a linter, there's a message from ale itself in ALEInfo (for example, Error: the command pylama could not be found within PATH or Pipfile's [scripts])... or an exit 1 with no output.

But yeah... I know is somewhat of a chicken-egg problem.