Memoize exception_lines in ExceptionPresenter
Memoize exception_lines in ExceptionPresenter

The method failure_lines uses exception_lines 3 times, which uses exception.message.
In some situation, exception.message can be slow (when inspect is called on a big object for example).
Without caching, this delay which can become considerable will be amplified by 3x.

rspec/rspec-core,rspec/rspec-core,rspec/rspec-core