Alerting: Update forking request handlers to use the same errors (#52965)

* generalize error handling in forking request handlers
* remove MatchesBackend and change test to test Can
* add 404 to route specs
* change backendTypeByUID to getDatasourceByUID of expected type
* use common errors in api testing
* handle 401 in errorToResponse
* replace backend type error with "unexpected datasource type"
* update swagger spec

pkg/services/ngalert/api/tooling/definitions/prom.go

@@ -19,6 +19,7 @@ import (
 //
 //     Responses:
 //       200: RuleResponse
+//       404: NotFound
 
 // swagger:route GET /api/prometheus/grafana/api/v1/alerts prometheus RouteGetGrafanaAlertStatuses
 //
@@ -33,6 +34,7 @@ import (
 //
 //     Responses:
 //       200: AlertResponse
+//       404: NotFound
 
 // swagger:model
 type RuleResponse struct {