Search query is entirely broken if one git repo generates a bad archive

Created by: slimsag

When https://github.com/sourcegraph/sourcegraph/issues/3798 occurred for me the search UI showed this error:

1 error occurred: * text search failed: failed to search 50.112.230.151/TEST/ssw-scientific--nndd@: failed to fetch missing archive cache item: failed to copy and close missing archive cache item: archive/tar: invalid tar header

What is surprising however is that despite the fact that the error only affects that one repository, it breaks the entire search and I get zero results:

type:file err

But omitting that repository from my query does give me results:

type:file err -repo:TEST\/ssw-scientific

This is a surprising bug because we have code and a UX in place to explicitly make it such that one bad repo does not ruin the entire search already. So the question is, why doesn't it work in this case?

To reproduce, you may either follow reproduction steps in #3798 (closed) (hard) OR simply change this method to always return an error for one specific repository on your dev instance: https://sourcegraph.com/github.com/sourcegraph/sourcegraph@95fb2bc/-/blob/cmd/searcher/main.go#L54

cc @sourcegraph/code-search