* Fix S3 backend test affected by making the Workspaces method return errors via diagnostics
* Address diagnostics comparison issues in test by ensuring expected diagnostics are defined in the context of the config they're triggered by
* Fix failing test case `TestBackendConfig_EC2MetadataEndpoint/envvar_invalid_mode` by making `diagnosticBase` struct comparable
* Add compile-time checks that diagnostic types fulfil interfaces
* Stop diagnosticBase implementing ComparableDiagnostic, re-add S3-specific comparer code to s3 package
* Update tests to use the S3-specific comparer again
* Fix test case missed in refactoring
* Update the backend.Backend interface to use diagnostics as return value from StateMgr method
* Fix calls to `Fatalf`
