Handle errors when default section(s) forgotten

[jaydesl]

Trying this one from a different angle...

Previously, if any of the default sections were omitted in a setup.cfg that defined a sections block, isort would throw an exception (see linked issues). Any omitted default sections are now appended to chain(sections, forced_separate) during parsing. Duplicates are avoided and the correct (default) order is maintained with an OrderedDict().

Fixes #1570 #1562

[timothycrosley]

Thanks for working on this! I'm wondering if it isn't better to hard fail in these cases, but just improve the documentation and presentation of the error message?

[anirudnits]

@timothycrosley I agree, it seems more of a documentation issue as discussed here #1562.

[jaydesl]

Here's another take:

• docs updated
• warn when default sections omitted in config file
• warn on each KeyError during parsing

I took to using warnings because I couldn't find a good example of a hard fail anywhere else in isort, but those warnings create a lot of noise.

Maybe a hard-fail similar to the catch-all is better?
eg

    except KeyError as error:
        if error.args[0] in DEFAULT_CONFIG.sections:
            printer = create_terminal_printer(color=config.color_output)
            printer.error(
                f"Parsing failed because the default {SECTION} is missing in `sections` config")\n"
                "Please read more here: https://pycqa.github.io/isort/#custom-sections-and-ordering"
            )
            raise

[jaydesl]

i.e something like this

[timothycrosley]

This looks pretty good to me! Thanks @jaydesl!