Fixes minor issues in the read() retries patch

Follow-up to commit c5b97eb781.

Two things slipped through the cracks:
* ImageDownloadError was instantiated incorrectly, resulting in a wrong
  error message. This was uncovered by using assertRaisesRegext in tests.
* We allowed calling write(None). This was uncovered by avoiding sleep(4)
  in tests and enabling more failed calls before timeout.

Change-Id: If5e798c5461ea3e474a153574b0db2da96f2dfa8
(cherry picked from commit 00ad03b709)

ironic_python_agent/extensions/standby.py

@@ -366,13 +366,14 @@ class ImageDownload(object):
             if chunk:
                 self._last_chunk_time = time.time()
                 self._hash_algo.update(chunk)
+                yield chunk
             elif (time.time() - self._last_chunk_time
                   > CONF.image_download_connection_timeout):
                 LOG.error('Timeout reached waiting for a chunk of data from '
                           'a remote server.')
                 raise errors.ImageDownloadError(
+                    self._image_info['id'],
                     'Timed out reading next chunk from webserver')
-            yield chunk
 
     def verify_image(self, image_location):
         """Verifies the checksum of the local images matches expectations.

ironic_python_agent/tests/unit/extensions/test_standby.py

@@ -1188,8 +1188,8 @@ class TestStandbyExtension(base.IronicAgentTest):
                 return self
 
             def __next__(self):
-                if self.count == 1:
-                    time.sleep(4)
+                if self.count:
+                    time.sleep(0.1)
                     return None
                 if self.count < 3:
                     self.count += 1
@@ -1211,8 +1211,9 @@ class TestStandbyExtension(base.IronicAgentTest):
         hexdigest_mock = md5_mock.return_value.hexdigest
         hexdigest_mock.return_value = image_info['checksum']
         requests_mock.side_effect = create_timeout
-        self.assertRaises(
+        self.assertRaisesRegex(
             errors.ImageDownloadError,
+            'Timed out reading next chunk',
             self.agent_extension._stream_raw_image_onto_device,
             image_info,
             '/dev/foo')