Add missing error handling to module_::def_submodule (#3973)

* Add missing error handling to module_::def_submodule

* Add test_def_submodule_failures

* PyPy only: Skip test with trigger for PyModule_GetName() failure.

* Reapply minor fix that accidentally got lost in transfer from PR #3964

tests/test_modules.cpp

@@ -120,4 +120,6 @@ TEST_SUBMODULE(modules, m) {
 
         return failures;
     });
+
+    m.def("def_submodule", [](py::module_ m, const char *name) { return m.def_submodule(name); });
 }