Handle ct_run exit codes in R15B02 and later

This is a new attempt of https://github.com/rebar/rebar/pull/42, which
should be backwards compatible with older versions.

Since R15B02, ct_run returns a non-zero exit code when some tests
failed or were auto-skipped. (See ticket OTP-9865.)

This fix makes it so a non-0 code doesn't cause an instant failure, but
still prompts for log verification before doing so. Given the behaviour
was acceptable for pre-R15B02, it should be valid with it with
post-R15B02.