V14: Return the unhealthy error message from Examine index

[elit0451]

Details

• Fixes the backend part of the following issue, reported in v14 - Examine Dashboard issues #16213:

When the IIndexDiagnostics reports unhealthy, the dashboard not longer uses the Attempt result to display the unhealthy error message. The string returned from this attempt should be displayed.

Test

• Change this line to the following:

Attempt<string?> isHealthy = Attempt.Fail("something went wrong");

• Verify that when an index is unhealthy, you get the "something went wrong" message using GET /umbraco/management/api/v1/indexer endpoint;
• Verify that the healthStatus::message (in the output) is null when there is no message, i.e. an index is healthy.