[Synthetics] Return 404 status code for monitor not found instead of 500 (elastic#238418)

## Summary

Return 404 status code for monitor not found instead of 500 , this is
especially broken for read-only user.

Author: shahzad31

x-pack/solutions/observability/plugins/synthetics/public/apps/synthetics/components/monitor_details/hooks/use_selected_monitor.tsx

@@ -15,7 +15,7 @@ import {
   selectEncryptedSyntheticsSavedMonitors,
   selectMonitorListState,
   selectorMonitorDetailsState,
-  selectorError,
+  selectSyntheticsMonitorError,
 } from '../../../state';
 import { useGetUrlParams } from '../../../hooks';
 
@@ -37,7 +37,7 @@ export const useSelectedMonitor = ({
     () => monitorsList.find((monitor) => monitor[ConfigKey.CONFIG_ID] === monitorId) ?? null,
     [monitorId, monitorsList]
   );
-  const error = useSelector(selectorError);
+  const error = useSelector(selectSyntheticsMonitorError);
   const { lastRefresh, refreshInterval } = useSyntheticsRefreshContext();
   const { syntheticsMonitor, syntheticsMonitorLoading, syntheticsMonitorDispatchedAt } =
     useSelector(selectorMonitorDetailsState);

x-pack/solutions/observability/plugins/synthetics/server/services/monitor_config_repository.ts

@@ -20,6 +20,7 @@ import type { EncryptedSavedObjectsClient } from '@kbn/encrypted-saved-objects-p
 import { withApmSpan } from '@kbn/apm-data-access-plugin/server/utils/with_apm_span';
 import { isEmpty, isEqual } from 'lodash';
 import type { Logger } from '@kbn/logging';
+import { SavedObjectsErrorHelpers } from '@kbn/core-saved-objects-server';
 import { MONITOR_SEARCH_FIELDS } from '../routes/common';
 import {
   legacyMonitorAttributes,
@@ -65,7 +66,10 @@ export class MonitorConfigRepository {
     ]);
     const resolved = results.saved_objects.find((obj) => obj?.attributes);
     if (!resolved) {
-      throw new Error('Monitor not found');
+      throw SavedObjectsErrorHelpers.createGenericNotFoundError(
+        syntheticsMonitorSavedObjectType,
+        id
+      );
     }
     return resolved;
   }