fix(#68): handle formatter errors

[barrettruth]

#68 warrants a bit more discussion about the exit code. While formatters exit with 0 (usually) on success, some linters like shellcheck exit with non-zero exit codes if they find errors. This means the following code, if put in spawn.lua, will not work if placed in function spawn() for some linters, but will resolve issue #68 by catching the exit code:

    if exit_code == 0 then
      coroutine.resume(co, table.concat(chunks))
    else
      on_failed(('process %s exited with non-zero exit code %s'):format(opt.cmd, exit_code))
      coroutine.resume(co)
      return
    end

As a result, I think a check_exit_code is necessary. I don't like to introduce complexity but it's either add the function or somehow distinguish between linters and formatters before spawn(), which sounds like it could get messy. thoughts @glepnir @xiaoshihou514 ?

BTW sorry for messing up previous PR - rebase skills are improving.

[glepnir]

usually exit code non 0 or stdout.len() < 999 is error. so if 0 is correct non 0 just on_failed. check_exit_code why need this function ?

[barrettruth]

Non-zero exit code does not indicate a failure always.

That logic falls apart for the linter shellcheck:
output is sent to stdout
non-zero exit code is returned when the linter runs correctly

How do you overcome this without check_exit_code function?

[xiaoshihou514]

@glepnir proposed the following

if (ret~=0 and #stderr>0) then
    -- determine that there was an error
else
    ...

which should cover most cases. Unless the tool is really messing with the convention, in which case the user should make use of the fn feature.

[barrettruth]

Ok - is this what you mean? @xiaoshihou514 .