Use `mrb_print_error` to print exception instead of `mrb_p`.

tasks/mrbgems_test.rake

@@ -55,7 +55,7 @@ MRuby.each_target do
               f.puts %Q[  mrb_load_irep(mrb2, gem_test_irep_#{g.funcname}_preload);]
             end
             f.puts %Q[  if (mrb2->exc) {]
-            f.puts %Q[    mrb_p(mrb2, mrb_obj_value(mrb2->exc));]
+            f.puts %Q[    mrb_print_error(mrb2);]
             f.puts %Q[    exit(EXIT_FAILURE);]
             f.puts %Q[  }]
             f.puts %Q[  mrb_const_set(mrb2, mrb_obj_value(mrb2->object_class), mrb_intern_lit(mrb2, "GEMNAME"), mrb_str_new(mrb2, "#{g.name}", #{g.name.length}));]
@@ -74,7 +74,7 @@ MRuby.each_target do
 
             f.puts %Q[  mrb_load_irep(mrb2, gem_test_irep_#{g.funcname}_#{i});]
             f.puts %Q[  if (mrb2->exc) {]
-            f.puts %Q[    mrb_p(mrb2, mrb_obj_value(mrb2->exc));]
+            f.puts %Q[    mrb_print_error(mrb2);]
             f.puts %Q[    exit(EXIT_FAILURE);]
             f.puts %Q[  }]
             f.puts %Q[  ]

test/init_mrbtest.c

@@ -16,7 +16,7 @@ mrb_init_mrbtest(mrb_state *mrb)
   mrbgemtest_init(mrb);
 #endif
   if (mrb->exc) {
-    mrb_p(mrb, mrb_obj_value(mrb->exc));
+    mrb_print_error(mrb);
     exit(EXIT_FAILURE);
   }
 }