api: use more generic error member for exceptions

Rename the `API.exception` member to `API.error`, to make it more
generic, so it can also be used for other sort of errors in the
future. Also add a layer of additional structure with `type` and
`data` members so different types of errors apart. Currently only
`exception` is used.
Adapt the tests in test/mod/test_api.py to check for the new
structure and its content.

osbuild/api.py

@@ -144,7 +144,7 @@ class API(BaseAPI):
         self._output_pipe = None
         self.monitor = monitor
         self.metadata = {}
-        self.exception = None
+        self.error = None
 
     @property
     def output(self):
@@ -187,7 +187,10 @@ class API(BaseAPI):
             sock.send({"type": "fd", "fd": 0}, fds=fds)
 
     def _get_exception(self, message):
-        self.exception = message["exception"]
+        self.error = {
+            "type": "exception",
+            "data": message["exception"],
+        }
 
     def _message(self, msg, fds, sock):
         if msg["method"] == 'add-metadata':

osbuild/pipeline.py

@@ -93,7 +93,7 @@ class Stage:
                                binds=[os.fspath(tree) + ":/run/osbuild/tree"],
                                readonly_binds=ro_binds)
 
-        return BuildResult(self, r.returncode, r.output, api.metadata, api.exception)
+        return BuildResult(self, r.returncode, r.output, api.metadata, api.error)
 
 
 class Assembler:
@@ -152,7 +152,7 @@ class Assembler:
                                binds=binds,
                                readonly_binds=ro_binds)
 
-        return BuildResult(self, r.returncode, r.output, api.metadata, api.exception)
+        return BuildResult(self, r.returncode, r.output, api.metadata, api.error)
 
 
 class Pipeline:

test/mod/test_api.py

@@ -96,8 +96,13 @@ class TestAPI(unittest.TestCase):
             p = mp.Process(target=exception, args=(path, ))
             p.start()
             p.join()
-        self.assertIsNotNone(api.exception, "Exception not set")
-        self.assertEqual(api.exception["value"], "osbuild test exception")
+        self.assertIsNotNone(api.error, "Error not set")
+        self.assertIn("type", api.error, "Error has no 'type' set")
+        self.assertEqual("exception", api.error["type"], "Not an exception")
+        e = api.error["data"]
+        for field in ("type", "value", "traceback"):
+            self.assertIn(field, e, f"Exception needs '{field}'")
+        self.assertEqual(e["value"], "osbuild test exception")
 
     def test_metadata(self):
         # Check that `api.metadata` leads to `API.metadata` being