[dashboard] invalid dashboard displayed as 404 instead of showing validation error

[nreese]

When loading a dashboard, any contentManagementService.client.get error is converted into a SavedObjectNotFound error. This results in 404 error message when that is not always the case. The dashboard may exist, and instead, the end point failed for another reason, like maybe the content management schema validation failed.

https://github.com/elastic/kibana/blob/main/src/platform/plugins/shared/dashboard/public/services/dashboard_content_management_service/lib/load_dashboard_state.ts#L87

const result = await contentManagementService.client
      .get<DashboardGetIn, DashboardGetOut>({
        contentTypeId: DASHBOARD_CONTENT_ID,
        id,
      })
      .catch((e) => {
        throw new SavedObjectNotFound(DASHBOARD_CONTENT_ID, id);
      });

Instead, only 404 errors should get converted to SavedObjectNotFound. The logic should be cleaned up like such

import { isHttpFetchError } from '@kbn/core-http-browser';
const result = await contentManagementService.client
      .get<DashboardGetIn, DashboardGetOut>({
        contentTypeId: DASHBOARD_CONTENT_ID,
        id,
      })
      .catch((e) => {
        if (isHttpFetchError(e) && e.response?.status === 404) {
          throw new SavedObjectNotFound(DASHBOARD_CONTENT_ID, id);
        }
        throw e;
      });

The problem is that DashboardRenderer only renders the text of the error as unformatted text. This should probably be clean-up and display something better. Maybe design needs to get involved.
https://github.com/elastic/kibana/blob/main/src/platform/plugins/shared/dashboard/public/dashboard_container/external_api/dashboard_renderer.tsx#L107

if (error) {
      return error instanceof SavedObjectNotFound ? (
        <Dashboard404Page dashboardRedirect={dashboardRedirect} />
      ) : (
        error.message
      );
    }

[elasticmachine]

Pinging @elastic/kibana-presentation (Team:Presentation)