Better feedback in case of errors.

[i-Zaak]

Regardless of the underlying issue, the pull from ssh remote often reports files missing in the cache ("Is your cache up to date?
https://error.dvc.org/missing-files"). This would be e.g. dvc-ssh not installed, ssh keys not unlocked (would prompt for password), or other reasons unrelated to the existence of files in cache. Running pull with verbose flags doesn't provide additional information.

It would be great if the error messages contained information relevant to the underlying problem.