The no-configuration error names one file, and four others are readable

Split out of [#576](https://github.com/akiomik/nostui/pull/576), which carried this and then removed it.

## What is there now

The error tells a fresh install to write `config.json`, and shows what to put in it. `Config::new` also reads `config.json5`, `config.yaml`, `config.toml` and `config.ini`. The README lists all five; the error names one.

## Why #576 stopped naming the rest

It did name them, and that list was where the change spent itself. Five reviews in a row found something in it:

- the four sat directly under `{"key": "nsec1..."}`, and two of them reject it — `config.toml` with that text is a parse error, `config.ini` answers `missing configuration field "key"`, which reads as having named the key wrong rather than having chosen the wrong file;
- splitting them into "take the same text" and "want their own syntax" left nothing asserting which name landed on which line, so swapping the two lists passed the whole suite;
- the predicate doing the splitting was itself unasserted — claiming TOML reads JSON passed too;
- its documentation described TOML as answering the way only INI does.

Each fix was right. Each one grew the thing that produced the next, and none of it was what #113 asked for.

## What would make it worth carrying

Not the names by themselves. A reader who wants YAML is already served by the README, and the error's job on a fresh install is to get one file written. What a list would have to add is the part the README does not: that the text shown does not travel between them.

That is a message-design question — whether the error is the place to say it, and what it costs in lines a first run has to read — rather than a bug. Worth answering before the code comes back, since the code is what kept being wrong about it.

## Acceptance

- If the error names more than one format, which of them accept the text it shows is asserted, and the assertion fails when the classification is wrong.
- The first run still reads in four lines or so.

Start at Config::new and the current no-configuration error, then compare the README’s five formats with the text shown on a fresh install. Decide whether the error should explain which formats accept that text; if it names more than one, assert each classification and keep the first-run message to about four lines.

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