Implement loading and error state in DAST scans view

What does this MR do and why?

This improves the on-demand scans UX by implementing a loading and an error states.

Screenshots or screen recordings

Loading state	Error state

How to set up and validate locally

1. Enable the on-demands scans page.

   Feature.enable(:dast_view_scans)

2. Visit any project's on-demand scans page such as http://127.0.0.1:3000/groups/flightjs/-/on_demand_scans.

   • If you have run at least one on-demand scan in the past, you should see the loading state here. Otherwise, follow the instructions to run an on-demand scan.

3. For the error state to show up, go wild and break the GraphQL query by applying the following patch (copy the code block and run pbpaste | git apply):

   diff --git a/ee/app/assets/javascripts/on_demand_scans/graphql/on_demand_scans.query.graphql b/ee/app/assets/javascripts/on_demand_scans/graphql/on_demand_scans.query.graphql
   index e6a5e1f4fbd..8b62bbbf645 100644
   --- a/ee/app/assets/javascripts/on_demand_scans/graphql/on_demand_scans.query.graphql
   +++ b/ee/app/assets/javascripts/on_demand_scans/graphql/on_demand_scans.query.graphql
   @@ -13,6 +13,7 @@ query allPipelinesCount($fullPath: ID!, $first: Int, $last: Int, $after: String,
            ...PageInfo
          }
          nodes {
   +        foo
            id
            path
            createdAt

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #341364 (closed)