Rely on #full_trace in ExceptionWrapper#source_extract

Exception#backtrace isn't the same as #full_trace. See
[rails/backtrace_cleaner.rb][]. We are leaking full path names, where we
used have them cleaned up.

Actually, renamed the method to #source_extracts because we return
multiple source extracts now. Its no longer a single source extract.

[rails/backtrace_cleaner.rb]: https://github.com/rails/rails/blob/master/railties/lib/rails/backtrace_cleaner.rb