Group-level dependencies: Add error handling for fetching projects for filter

The project filters on the group-level dependencies app fetch a list of projects. Currently, there is no error handling in place and if anything goes wrong during the fetch, this would just happen without informing the user of what went wrong.

This issue is to capture adding the handling.

Verification:

1. Navigate to https://gitlab.com/groups/gitlab-org/govern/threat-insights-demos/verification-projects/verify-408846-group/-/dependencies
2. Wait until the page has loaded
3. Open your browsers DevTools and enable offline emulation (eg.: Chrome: Network -> Throttling -> offline).
4. Click on "Search or filter dependencies ..."
5. Choose "Project"
6. Verify that an error banner pops up, displaying, "There was an error fetching the projects for this group. Please try again later."
7. Note: Because we emulated being offline, the error icon within the banner might not show up. This would not happen in most real-life scenarios, since the user would still be online and the icon would load.