Improve logic that finds the failure line to print.

Created by: myronmarston

• Introduce project_source_dirs config setting.
• Look for the first backtrace line in one of the project_source_dirs rather than the first line from your spec file. This helps in situations where you define a helper method in a support file that has a failing expectation, and call it from your spec. Previously it would have shown the helper method call site rather than the expectation in the helper method itself.
• If no backtrace line can be found in a project_source_dirs, pick the first backtrace line. While we don’t generally want to show lines from gems, it’s better than showing no line at all.

Fixes #1991 (closed).