errors: introduce StateError and use it from commands and cmdutil
This very similar to an earlier patch (which was for `InputError`).
In this patch, I also updated the transplant extension only because
`test-transplant.t` would otherwise have needed a `#if continueflag`.
Differential Revision: https://phab.mercurial-scm.org/D9310
[fix]
clang-format:command = clang-format --style file
clang-format:pattern = set:(**.c or **.cc or **.h) and not "include:contrib/clang-format-ignorelist"
rustfmt:command = rustfmt +nightly
rustfmt:pattern = set:"**.rs" - "mercurial/thirdparty/**"
black:command = black --config=black.toml -
black:pattern = set:**.py - mercurial/thirdparty/**
# Mercurial doesn't have any Go code, but if we did this is how we
# would configure `hg fix` for Go:
go:command = gofmt
go:pattern = set:**.go