Issues when requesting a backup don't provide error information

[Charles-Gagnon]

Currently the error information provided when there's issues requesting a backup is minimal or nonexistent.

For example, try to request a backup (using BackupProvider.backup(...) from ADS for example) and give it an invalid connection URI. The call succeeds and returns a BackupResponse with a Result of false, but that's it. No error message or anything indicating what the issue was.

Note that this is mostly related to the setup being done before the task is started, if there's an issue when the task is running then the task system will report that as failed as expected.

The logic in the request handlers is the issue here - for the example above it correctly checks whether backup is "supported" but then nothing it does gives any further information about what went wrong. So if the URI was invalid, or the connection was an Azure connection for example.

https://github.com/microsoft/sqltoolsservice/blob/main/src/Microsoft.SqlTools.ServiceLayer/DisasterRecovery/DisasterRecoveryService.cs#L330

We should be giving as much information as possible back to the user - especially for extensibility APIs like this.