api: properly encode exception type

When using `str(type(exception))` this ends up to be something like
`<class 'ValueError'>` for a `ValueError` exception. Get the vanilla
name of the exception type via `type(exception).__name__`.
Add a test to ensure that we encode this properly.

test/mod/test_api.py

@@ -103,6 +103,7 @@ class TestAPI(unittest.TestCase):
         for field in ("type", "value", "traceback"):
             self.assertIn(field, e, f"Exception needs '{field}'")
         self.assertEqual(e["value"], "osbuild test exception")
+        self.assertEqual(e["type"], "ValueError")
 
     def test_metadata(self):
         # Check that `api.metadata` leads to `API.metadata` being