Improve messaging for errors coming from go-git-providers

[makkes]

When go-git-providers isn't able to fetch repository details while bootstrapping it's hard to understand where the error has originated. Here is an error caused by the repository not being reachable because of missing proxy settings:

flux bootstrap github --owner=bar --repository=baz --branch=master --hostname=foo --token-auth --private=true --verbose --log-level=debug
connecting to foo
failed to get Git repository "https://foo/bar/baz": Get "https://foo/api/v3/bar/baz": Forbidden

We should prefix errors coming from go-git-providers with provider error: %w so it's easier to debug the cause.