swift

2ac84602 - [Backtracing] Massage the output a little in various cases.

[Backtracing] Massage the output a little in various cases. We shouldn't try to erase the message when in non-color mode; mostly that's intended for redirected output scenarios anyway, and in that case we don't really want garbage in the output. Also, improve the failure messages when the backtracer itself goes wrong or can't be executed. Finally, change the behaviour slightly so that *if* we're explicitly enabled, *and* the backtracer path wasn't explicitly specified, *and* we can't find the backtracer, we print a warning on start-up. We don't do that in any other case because we don't want spurious warnings everywhere. rdar://118055527