Do not discard amended format when f-interpolator warns

[som-snytt]

Also use the same error of kind Interpolation for warnings in s.

There are assorted errors that f emits, and a handful of warnings (about format string edge cases).

Previously, f would discard the amended "parts" and adapted "args" on any diagnostic; the transform would just call format directly, so the output looks normal. But the toString warning revealed that it loses the "injected" %s specifier; that would be witnessed only if one of the rare "edge case" warnings happened to be emitted. Now it does not discard on warnings, only on hard errors.

Fixes #23693

[som-snytt]

Note to self: it would have been easier to see what was going on if FormatInterpolatorTransform were inlined in StringInterpolatorOpt#transformF. It's confusing that Transform.checked dispatches to FormatChecker#checked. I see the components were confusing before the refactor or forward port, but it could be simpler; that is, extra layers harms local reasoning. It's also nice when names are different.

Edit: went ahead and inlined that code. The other suboptimal thing is "string is empty" means "error".