Ignore failure to create existing directory

[bcc32]

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Before submitting a pull request make sure you have:

• Searched the bugtracker for similar pull requests
• Read adding new extractor tutorial
• Read youtube-dl coding conventions and adjusted the code to meet them
• Covered the code with tests (note that PRs without tests will be REJECTED)
• Checked the code with flake8

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• I am the original author of this code and I am willing to release it under Unlicense
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What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

Fix a bug where a video would not be downloaded because creating the destination folder failed due to already existing. This bug can happen when multiple videos are being downloaded to the same destination folder by concurrent youtube-dl processes (e.g., through GNU parallel).

I did not add tests because the fix is for a heisenbug that, by definition, is not possible to write a deterministic test for.