tests/test-check-rust-format.t

errors: make OutOfBandError extend Abort

I'm about to create a new `RemoteError` exception and make
`OutOfBandError` extend it. This patch prepares for that.

Differential Revision: https://phab.mercurial-scm.org/D10465

#require rustfmt test-repo

  $ . "$TESTDIR/helpers-testrepo.sh"

  $ cd "$TESTDIR"/..
  $ RUSTFMT=$(rustup which --toolchain nightly-2020-10-04 rustfmt)
  $ for f in `testrepohg files 'glob:**/*.rs'` ; do
  >   $RUSTFMT --check --edition=2018 --unstable-features --color=never $f
  > done