The existing wording was confusing for this situation:

* You have two specs, a and b.
* Spec a always fails.
* Spec b fails only if a runs first.

In such a situation, your failures are order dependent,
but our bisector printed "failure is not order dependent",
which was quite confusing. The new wording,
"failures(s) do not require any non-failures to run first"
is more accurate.

`Exception.new.backtrace` is `nil` so we can't count on
there always being a backtrace. Before now we relied
on the `format_backtrace` caller handling this (passing
`ex.backtrace || []`), but with #2074 now also including
the `exception.cause` in failure output, the code there
did not guard the `format_backtrace` call with `|| []`
and is causing users to get a `NoMethodError`. It's
easiest if `format_backtrace just handles the `nil` case
so each caller does not have to remember to handle it.

Fixes #2117.