Refactor to replace Kernel.#exit with custom error (rubocop#5386)

Handle exit status consistently in `RuboCop::CLI#run` method.

lib/rubocop/cli.rb

@@ -29,13 +29,16 @@ def initialize
     # @param args [Array<String>] command line arguments
     # @return [Integer] UNIX exit code
     #
-    # rubocop:disable Metrics/MethodLength
+    # rubocop:disable Metrics/MethodLength, Metrics/AbcSize
     def run(args = ARGV)
       @options, paths = Options.new.parse(args)
       validate_options_vs_config
       act_on_options
       apply_default_formatter
       execute_runners(paths)
+    rescue RuboCop::ConfigNotFoundError => e
+      warn e.message
+      return e.status
     rescue RuboCop::Error => e
       warn Rainbow("Error: #{e.message}").red
       return 2
@@ -49,7 +52,7 @@ def run(args = ARGV)
       warn e.backtrace
       return 2
     end
-    # rubocop:enable Metrics/MethodLength
+    # rubocop:enable Metrics/MethodLength, Metrics/AbcSize
 
     def trap_interrupt(runner)
       Signal.trap('INT') do

lib/rubocop/config_loader.rb

@@ -4,6 +4,16 @@
 require 'pathname'
 
 module RuboCop
+  # Raised when a RuboCop configuration file is not found.
+  class ConfigNotFoundError < Error
+    attr_reader :status
+
+    def initialize(path, status)
+      super("Configuration file not found: #{path}")
+      @status = status
+    end
+  end
+
   # This class represents the configuration of the RuboCop application
   # and all its cops. A Config is associated with a YAML configuration
   # file from which it was read. Several different Configs can be used
@@ -167,9 +177,8 @@ def load_yaml_configuration(absolute_path)
       # found" error.
       def read_file(absolute_path)
         IO.read(absolute_path, encoding: Encoding::UTF_8)
-      rescue Errno::ENOENT
-        warn(format('Configuration file not found: %s', absolute_path))
-        exit(Errno::ENOENT::Errno)
+      rescue Errno::ENOENT => e
+        raise ConfigNotFoundError.new(absolute_path, e.errno)
       end
 
       def yaml_safe_load(yaml_code, filename)

spec/rubocop/config_loader_spec.rb

@@ -827,11 +827,10 @@ def cop_enabled?(cop_class)
 
       it 'prints a friendly (concise) message to stderr and exits' do
         expect { load_file }.to(
-          output(/Configuration file not found/).to_stderr.and(
-            raise_error(SystemExit) do |e|
-              expect(e.status).to(eq(Errno::ENOENT::Errno))
-            end
-          )
+          raise_error(RuboCop::ConfigNotFoundError) do |e|
+            expect(e.status).to(eq(Errno::ENOENT::Errno))
+            expect(e.message).to(match(/\AConfiguration file not found: .+\z/))
+          end
         )
       end
     end