Improve error message by including the error code

src/python/grpcio/grpc/_cython/_cygrpc/aio/call.pyx.pxi

@@ -125,7 +125,7 @@ cdef class _AioCall(GrpcCallWrapper):
         if credentials is not None:
             set_credentials_error = grpc_call_set_credentials(self.call, credentials.c())
             if set_credentials_error != GRPC_CALL_OK:
-                raise RuntimeError("Credentials couldn't have been set")
+                raise RuntimeError(f"Credentials couldn't have been set: {set_credentials_error}")
 
         grpc_slice_unref(method_slice)
 

src/python/grpcio_tests/tests_aio/unit/_test_server.py

@@ -24,6 +24,10 @@ from tests_aio.unit import _constants
 
 _INITIAL_METADATA_KEY = "x-grpc-test-echo-initial"
 _TRAILING_METADATA_KEY = "x-grpc-test-echo-trailing-bin"
+_PRIVATE_KEY = resources.private_key()
+_CERTIFICATE_CHAIN = resources.certificate_chain()
+_TEST_ROOT_CERTIFICATES = resources.test_root_certificates()
+_SERVER_CERTS = ((_PRIVATE_KEY, _CERTIFICATE_CHAIN),)
 
 
 async def _maybe_echo_metadata(servicer_context):
@@ -38,11 +42,6 @@ async def _maybe_echo_metadata(servicer_context):
                               invocation_metadata[_TRAILING_METADATA_KEY])
         servicer_context.set_trailing_metadata((trailing_metadatum,))
 
-_PRIVATE_KEY = resources.private_key()
-_CERTIFICATE_CHAIN = resources.certificate_chain()
-_TEST_ROOT_CERTIFICATES = resources.test_root_certificates()
-_SERVER_CERTS = ((_PRIVATE_KEY, _CERTIFICATE_CHAIN),)
-
 
 async def _maybe_echo_status(request: messages_pb2.SimpleRequest,
                              servicer_context):