Followups from #2065.

Created by: myronmarston

• Better warning when an invalid value is passed for --fail-fast (including silencing call site since it will not be in user code).
• No need to stub reporter.failures_required.
• No need for the failures_required method at all.
• Use fail_fast_limit_met? in more situations.
• Remove unneeded ExampleGroup.fail_fast? method.