Fix error paths of lldb rbbt

Without an argument, or with an expression that failed to evaluate, rbbt raised a Python exception instead of reporting the problem. Evaluating without a process referred to an undefined `target` as well.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

misc/lldb_cruby.py

@@ -527,14 +527,15 @@ def rb_backtrace(debugger, command, result, internal_dict):
         if frame.IsValid():
             val = frame.EvaluateExpression(command)
         else:
-            val = target.EvaluateExpression(command)
+            val = bt.target.EvaluateExpression(command)
 
         error = val.GetError()
         if error.Fail():
-            print >> result, error
+            print(error, file=result)
             return
     else:
-        print("Need an EC for now")
+        print("Need an EC for now", file=result)
+        return
 
     bt.print_bt(val)
 

misc/test_lldb_cruby.rb

@@ -14,8 +14,9 @@ run #{ruby_opts} -e'#{code}'
 #{command}
 eom
       tf.flush
-      o, s = Open3.capture2('lldb', '-b', '-s', tf.path)
+      o, e, s = Open3.capture3('lldb', '-b', '-s', tf.path)
       assert_true s.success?, message
+      assert_not_match /^Traceback/, e, message
       assert_match /^\(lldb\) #{Regexp.quote(command)}\n#{pattern}/, o, message
     end
   end
@@ -47,6 +48,11 @@ eom
                 /rb_control_frame_t +TYPE *\n0x\h+ +EVAL +-e <main>\n0x\h+ +METHOD +-e foo\n0x\h+ +CFUNC *\n/
   end
 
+  def test_rbbt_error
+    assert_lldb 'p 1', 'rbbt', /Need an EC for now\n/
+    assert_lldb 'p 1', 'rbbt no_such_variable', /(?:.*\n)*?error: .*no_such_variable/
+  end
+
   def test_rbbt_zjit
     omit 'ZJIT is not built' unless system('./miniruby', '--zjit', '-e', 'exit RubyVM::ZJIT.enabled?', err: File::NULL)
     # Call foo deeper than the warm-up so that bar lands on a frame whose _iseq no interpreter has written