Surfaces errors from namespace delete properly

[mikenomitch]

Adds a "return err" that was accidentally dropped shortly after the Namespace OSS migration.

fixes #19414

[mikenomitch]

Note: I still need to test this manually. I will update this comment once I do.

EDIT: Have now manually tested and confirmed that the error is bubbling up to the API response - see below.

[mikenomitch]

Have now manually tested this and it works (with a soon-to-come UI change in another PR):

Thanks for the review - on those changes!

[jrasell]

LGTM with wording suggestion on the changelog.

If we really wanted to we could also add a test to ensure non-default namespace deletions return an error; but I assume this is also caught by state store testing.

[jrasell]

Suggested change

	namespaces: Failed delete calls no longer return success codes
	namespaces: Fixed a bug where a 200 OK response was returned even when the namespace delete failed

[tgross]

LGTM!