PyImport_ExecCodeModuleEx(): remove module from sys.modules in error cases.

PyImport_ExecCodeModuleEx():  remove module from sys.modules in error cases.
PyImport_ReloadModule():  restore the module to sys.modules in error cases.
load_package():  semantic-neutral refactoring from an earlier stab at
                 this patch; giving it a common error exit made the code
                 easier to follow, so retaining that part.
_RemoveModule():  new little utility to delete a key from sys.modules.