chore: cleaner fetch paths for slugs

[bmschmidt]

Suggested changes to dataset fetching. Goals are:

1. Preserve current behavior;
2. Add deprecation warning to the the behavior that depends on passing slugs without an org name, because this magic keeps biting people.
3. Throw http errors directly instead of assuming things couldn't be found.

[bmschmidt]

• chore: cleaner fetch paths for slugs #371 👈 (View in Graphite)
• main

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