Report Widgets Core: Add support for fetch failures

Context

For &7066 (closed) we need to display an error message when fetchCollapsedData fails.

The widget base does not provide an easy hook / message to display a failure message or handle the error.

Required reading

Please ensure that:

• all the code written implements the rules of Accessibility https://design.gitlab.com/regions/merge-request-reports#accessibility
• you're familiar with the overall goal https://www.figma.com/file/qEddyqCrI7kPSBjGmwkZzQ/Component-library?node-id=38193%3A30482

Task

Add support for rendering a failure message when a fetch fails. Possible solutions:

• Add a i18n.failure option.
• Add a hook to call when fetching fails with the error.