`brew bundle exec` behaves correctly only after doing `brew bundle install`.
Running `brew bundle check` can be slow, so let's add a `--check` flag
to `brew bundle exec` which will also run `brew bundle check` before
`brew bundle exec` to ensure that the `Brewfile` has been installed
before proceeding.
If callers of `brew bundle check` have already output some formulae
errors, they can set the
`HOMEBREW_BUNDLE_CHECK_ALREADY_OUTPUT_FORMULAE_ERRORS` environment
variable to the names of the formulae that have already been output.
