api: properly serialize the exception's traceback

Use `traceback.print_tb()` to serialize the exceptions' backtrace.
The previously used expression `str(e.__traceback__)` will just
give `<traceback object at 0x…>`, which is not very helpful.
Add a test to check that the method name that raises the exception,
also called `exception`, is in the traceback.

osbuild/api.py

@@ -210,12 +210,15 @@ def exception(e, path="/run/osbuild/api/osbuild"):
     """Send exception to osbuild"""
     traceback.print_exception(type(e), e, e.__traceback__, file=sys.stderr)
     with jsoncomm.Socket.new_client(path) as client:
+        with io.StringIO() as out:
+            traceback.print_tb(e.__traceback__, file=out)
+            stacktrace = out.getvalue()
         msg = {
             "method": "exception",
             "exception": {
                 "type": type(e).__name__,
                 "value": str(e),
-                "traceback": str(e.__traceback__)
+                "traceback": stacktrace
             }
         }
         client.send(msg)

test/mod/test_api.py

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