More user friendly output when no script specified

Fixes #256.

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Master (Unreleased)
 
+### Fixed
+
+* Unfriendly output in byebug's executable when no script specified (#256).
+
 ## 9.0.0 - 2016-05-11
 
 ### Fixed

lib/byebug/runner.rb

@@ -13,11 +13,6 @@ module Byebug
   class Runner
     include Helpers::ParseHelper
 
-    #
-    # Error class signaling absence of a script to debug.
-    #
-    class NoScript < StandardError; end
-
     #
     # Error class signaling a non existent script to debug.
     #
@@ -105,6 +100,8 @@ def run
         return
       end
 
+      return if no_script?
+
       Byebug.run_init_script if init_script
 
       setup_cmd_line_args
@@ -135,14 +132,23 @@ def option_parser
       end
     end
 
+    #
+    # No script to debug specified
+    #
+    def no_script?
+      return false unless $ARGV.empty?
+
+      interface.errmsg('You must specify a program to debug')
+      interface.puts(option_parser.help)
+      true
+    end
+
     #
     # Extracts debugged program from command line args.
     #
     def setup_cmd_line_args
       Byebug.mode = :standalone
 
-      raise(NoScript, 'You must specify a program to debug...') if $ARGV.empty?
-
       program = which($ARGV.shift)
       program = which($ARGV.shift) if program == which('ruby')
       raise(NonExistentScript, "The script doesn't exist") unless program

test/runner_test.rb

@@ -47,7 +47,9 @@ def test_run_with_remote_option_with_host_and_port_specification
 
     def test_run_without_a_script_to_debug
       with_command_line('bin/byebug') do
-        assert_raises(Runner::NoScript) { runner.run }
+        runner.run
+
+        check_error_includes 'You must specify a program to debug'
       end
     end