Prevent warnings if SDO setup fails

[sveinse]

The test test_block_download_not_supported is creating warnings from pytest

test/test_local.py::TestSDO::test_block_download_not_supported
  C:\Svein\canopen\venv\Lib\site-packages\_pytest\unraisableexception.py:85: PytestUnraisableExceptionWarning: Exception ignored in: <canopen.sdo.client.BlockDownloadStream object at 0x000001DA52BA6350>

  canopen.sdo.exceptions.SdoAbortedError: Code 0x05040001, Unknown SDO command specified

  During handling of the above exception, another exception occurred:

  Traceback (most recent call last):
    File "C:\Svein\canopen\canopen\sdo\client.py", line 794, in close
      if self.crc_supported:
         ^^^^^^^^^^^^^^^^^^
  AttributeError: 'BlockDownloadStream' object has no attribute 'crc_supported'

canopen/test/test_local.py

Lines 48 to 55 in 7ddb19b

	def test_block_download_not_supported(self):
	data = b"TEST DEVICE"
	with self.assertRaises(canopen.SdoAbortedError) as context:
	with self.remote_node.sdo[0x1008].open('wb',
	size=len(data),
	block_transfer=True) as fp:
	pass
	self.assertEqual(context.exception.code, 0x05040001)

This PR fixes the issue. The problem was 2-fold.

• The test tries to use a block download, which SdoServer.block_download() handles by sending an SDO abort. However, sending the abort fails, because SdoServer.abort() requires self._index and self._subindex.
• The SdoClient.open() opens a BlockDownloadStream(), but it doesn't get to initialize fully before the effort is aborted. When the object is closed, it fails as it's left in limbo.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
canopen/sdo/client.py	50.00%	1 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!

[acolomb]

Nice find and looks like an appropriate fix. Just one minor, optional suggestion.

[acolomb]

Seems like merging was allowed for you in this case. I didn't change anything though since the last PR. Hope it stays OK now.