[user errors] compulsory case names, allow multiple (#110878)
We want to get to a point where most UserErrors link to exportdb examples. This PR makes passing case names non-optional to make this intent clearer and encourage developers who raise UserErrors to make or point to examples that make fixing such errors more obvious for users.
In addition, sometimes there are multiple examples that are relevant to an error. Thus this PR also enables passing multiple case names.
Retry of #110733 which was reverted due to a landrace.
Differential Revision: [D50087148](https://our.internmc.facebook.com/intern/diff/D50087148/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/110878
Approved by: https://github.com/gmagogsfm, https://github.com/tugsbayasgalan
