Fix build & address review comments

Fixes https://github.com/grpc/grpc/issues/20532

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: {0}".format(set_credentials_error))
+                raise InternalError("Credentials couldn't have been set: {0}".format(set_credentials_error))
 
         grpc_slice_unref(method_slice)
 

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

@@ -125,7 +125,7 @@ cdef class AioChannel:
         cdef CallCredentials cython_call_credentials
         if python_call_credentials is not None:
             if not self._is_secure:
-                raise RuntimeError("Call credentials are only valid on secure channels")
+                raise UsageError("Call credentials are only valid on secure channels")
 
             cython_call_credentials = python_call_credentials._credentials
         else:

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

@@ -1,3 +1,18 @@
+# Copyright 2020 The gRPC Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""Tests the behaviour of the Call classes under a secure channel."""
+
 import unittest
 import logging
 
@@ -89,11 +104,12 @@ _STREAM_OUTPUT_REQUEST_ONE_RESPONSE.response_parameters.append(
 
 
 class TestStreamStreamSecureCall(_SecureCallMixin, AioTestBase):
+    _STREAM_ITERATIONS = 2
 
     async def test_async_generator_secure_channel(self):
 
         async def request_generator():
-            for _ in range(2):
+            for _ in range(self._STREAM_ITERATIONS):
                 yield _STREAM_OUTPUT_REQUEST_ONE_RESPONSE
 
         call_credentials = grpc.composite_call_credentials(