fix: returned 404 if feed does not exist

[jcpitre]

Summary:
Closes #600

Applies to /v1/gtfs_feeds/{id}/datasets endpoint
Checked for the existence of a feed before trying to extract its datasets.

Expected behavior:

If the feed does not exist it should return 404
If the feed does exist but has no datasets, it should return 200 with an empty list.

Testing tips:

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