fix(bundles): handle load failures gracefully

[mrcjkb]

If a bundle fails to load (e.g. because a rock is not yet installed), rocks-config will panic. This in turn can prevent rocks.nvim extensions from being loaded (like rocks-git.nvim), thus preventing the bundle from being installable - a vicious cycle.

This PR changes the logic of loading bundles to be in line with the logic of loading individual plugin configs:

• Fail gracefully
• Store the bundle and error so that it can be retrieved with :checkhealth rocks-config.

[mrcjkb]

Merging as this is covered by tests.