Don't cache Endpoints if a source throws

[halter73]

Description

Don't cache Endpoints in CompositeEndpointDataSource if a child EndpointDataSource source throws. CompositeEndpointDataSource is almost always the root EndpointDataSource, so this affects most apps.

Fixes #43693

Customer Impact

Without this fix, you do not see endpoint generation errors after the first request. You should see something like:

But instead get 404s after the first failed request:

We should be consistently throwing exceptions to outer middleware like the one responsible for the developer exception page and producing 500s instead of 404s.

Regression?

• Yes
• No

This is a regression introduced in 7.0-preview6 by #42195.

Risk

• High
• Medium
• Low

This is merely waiting to cache a list later after we know nothing threw while generating the list's elements.

Verification

• Manual (required)
• Automated

Packaging changes reviewed?

• Yes
• No
• N/A

@Pilchie

[ghost]

Hi @halter73. Please make sure you've updated the PR description to use the Shiproom Template. Also, make sure this PR is not marked as a draft and is ready-to-merge.

To learn more about how to prepare a servicing PR click here.

[Pilchie]

Approved for .NET 7 RC2.