check: improve output if repository is damaged

[MichaelEischer]

What does this PR change? What problem does it solve?

See the individual commits for details, the main changes are the following:

• Also report repository contains errors if the index cannot be loaded
• Guide users to the troubleshooting docs if an error was found
• additional files were found in the repo is only shown if there is no actual error. This message has proven to be rather confusing to users. Thus hide it if there's a bigger problem.
• The orphaned pack files warnings are now also only visible for check -v.
• Also suggest using repair pack if check (without --read-data) detects truncated pack files.
• Slightly expand docs on check errors

Was the change previously discussed in an issue or on the forum?

Fixes https://github.com/restic/restic/issues/4801

Checklist

• [x] I have read the contribution guidelines.
• [x] I have enabled maintainer edits.
• [ ] I have added tests for all code changes.
• [x] I have added documentation for relevant changes (in the manual).
• [x] There's a new file in changelog/unreleased/ that describes the changes for our users (see template).
• [x] I have run gofmt on the code in all commits.
• [x] All commit messages are formatted in the same style as the other commits in the repo.
• [x] I'm done! This pull request is ready for review.