Add missing test for check_all_source_directories_exist() raising.

tests/unit/borg/test_create.py

@@ -2580,3 +2580,11 @@ def test_check_all_source_directories_exist_with_glob_and_tilde_directories():
     flexmock(module.os.path).should_receive('exists').with_args('/root/bar').and_return(True)
 
     module.check_all_source_directories_exist(['foo*', '~/bar'])
+
+
+def test_check_all_source_directories_exist_with_non_existent_directory_raises():
+    flexmock(module).should_receive('expand_directory').with_args('foo').and_return(('foo',))
+    flexmock(module.os.path).should_receive('exists').and_return(False)
+
+    with pytest.raises(ValueError):
+        module.check_all_source_directories_exist(['foo'])