plugins: Should beets exit if plugins are not found or fail to load?

This came up in https://github.com/beetbox/beets/pull/5739#discussion_r2050456386:

Currently, we're not really consistent, for example aborting if plugin init fails, but not if the plugin has syntax errors.

In my opionion, we should simply always exit with a human-readable error, since this has the potential to mess up imports (for example due to missing stages) in hard-to-fix ways. But as shown by the linked discussion, there might be different opinions on how beets should behave.