Add the instance variable name and the module in Ractor::IsolationError (ruby#15563)

Author: etiennebarrie

bootstraptest/test_ractor.rb

@@ -846,7 +846,7 @@ def ractor_local_globals
 }
 
 # ivar in shareable-objects are not allowed to access from non-main Ractor
-assert_equal "can not get unshareable values from instance variables of classes/modules from non-main Ractors", <<~'RUBY', frozen_string_literal: false
+assert_equal "can not get unshareable values from instance variables of classes/modules from non-main Ractors (@iv from C)", <<~'RUBY', frozen_string_literal: false
   class C
     @iv = 'str'
   end

variable.c

@@ -1447,7 +1447,8 @@ rb_ivar_lookup(VALUE obj, ID id, VALUE undef)
                     UNLIKELY(!rb_ractor_main_p()) &&
                     !rb_ractor_shareable_p(val)) {
                 rb_raise(rb_eRactorIsolationError,
-                        "can not get unshareable values from instance variables of classes/modules from non-main Ractors");
+                        "can not get unshareable values from instance variables of classes/modules from non-main Ractors (%"PRIsVALUE" from %"PRIsVALUE")",
+                        rb_id2str(id), obj);
             }
             return val;
         }