Remove Ruby warnings

This PR includes two improvements:

1. `lib/simplecov/jruby16_fix.rb:31: warning: assigned but unused variable - e`

    The previous catch-all `rescue => e` was used to swallow this exception:
    `Errno::ENOENT: No such file or directory - test/fixtures/generated_buddha.rb`
    We now make sure the file is present with `File.file?`.
                      
    Also reverts (partially) 6f711098dbe3dccac067f1a128fc33ef2f845079 and moves the fix into jruby_fix.rb to keep JRuby fixes in the same file.

2. `lib/simplecov/defaults:57: warning: instance variable @exit_status not initialized`

   We now ensure that `@exit_status` is always initialized.

   Also did some cosmetic arrangements with no functional changes.