Packages: gracefully handle 403 errors when fetching packages
Created by: olafurpg
Many Rust crates are currently failing to "clone" due to 403 errors https://sourcegraph.com/site-admin/repositories?status=failed-fetch
These crates have in common that they don't exist on crates.io, for example https://crates.io/crates/alloc
We should gracefully handle errors like this so that the Sourcegraph repos don't have a "failed fetch" status. Candidates solutions include
- silently ignore 403 errors
- create the repos with a readme.md that includes the error message, this would make it easier for site admins to troubleshoot the issue
- delete the crate versions from
lsif_dependency_reposso that we stop trying to fetch the crate versions that error

