Cleanup warnings

When Ruby's `-w` is given (e.g. debugging Rails), pry will output these warnings and they are too annoying:

 * `lib/pry/code.rb:271: warning: instance variable @with_marker not initialized`
 * `lib/pry/commands/show_info.rb:93: warning: instance variable @used_super not initialized`
 * `lib/pry/commands/whereami.rb:140: warning: instance variable @method_code not initialized`
 * `lib/pry/history.rb:86: warning: File.exists? is a deprecated name, use File.exist? instead`
 * `lib/pry/module_candidate.rb:61: warning: instance variable @source not initialized`
 * `lib/pry/module_candidate.rb:79: warning: instance variable @source_location not initialized`
 * `lib/pry/pager.rb:137: warning: instance variable @system_pager not initialized`
 * `lib/pry/pager.rb:186: warning: instance variable @pager not initialized`
 * `lib/pry/wrapped_module.rb:311: warning: instance variable @all_source_locations_by_popularity not initialized`
This method is still using `defined?` since this is a class method and it describes what it checks better than `nil?`. If there was a case where `$?.success?` could return `nil` the behaviour would change, but I think it's ok unless we use an asynchronous cmd.