fs: repo: raise proper FileNotFoundError in metadata

[pared]

Related: #7120 #7157

• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

[pared]

Example and reproduction steps are described in #7120

In the example provided by me in #7120 (comment):

was:

$ dvc list .                                                                                   2.000s 
ERROR: unexpected error

after this change:

$ dvc list .                                                                                   0.000s 
ERROR: unexpected error - [Errno 2] No such file or directory: '/private/var/folders/f2/zs40fvgd4kd3z8jtzd9k0dxw0000gn/T/test_wspace/test_repo/link'

[pared]

It seems to me that there are few places where we are raising empty generic exceptions. We should probably go over those and add some context.