raise specific exception when preconditions fail

Improves upon the fix for  #146

lib/gli/app_support.rb

@@ -79,7 +79,7 @@ def run(args) #:nodoc:
           call_command(parsing_result) 
           0
         else
-          exit_now!(nil,65)
+          raise PreconditionFailed, "preconditions failed"
         end
       rescue Exception => ex
         if the_command.nil? && ex.respond_to?(:command_in_context)

lib/gli/exceptions.rb

@@ -11,6 +11,11 @@ class BadCommandLine < StandardError
     def exit_code; 64; end
   end
 
+  class PreconditionFailed < StandardError
+    include StandardException
+    def exit_code; 65; end
+  end
+
   # Indicates the bad command line was an unknown command
   class UnknownCommand < BadCommandLine
   end

test/tc_command.rb

@@ -355,7 +355,7 @@ def test_pre_exiting_false_causes_nonzero_exit
     @app.pre { |*| false }
 
     assert_equal 65,@app.run(["bs"]) # BSD for "input data incorrect in some way"
-    assert_equal '',@fake_stderr.to_s
+    assert_equal 'error: preconditions failed',@fake_stderr.to_s
     assert_equal '',@fake_stdout.to_s
   end