Go: handle missing import error and submodule cloning failure error

[jakecoffman]

Continuing the effort from #6730 to categorize known errors that we can't fix.

I'm seeing "package \S+ is not in GOROOT" quite a bit which indicates a relative import path that is not present. For instance, the package is named web and there's an import web/package where the directory package doesn't exist. This might be because they have generated code that isn't checked in, which is against Go conventions, so I don't think we need to try to support it:

once the file is generated (and tested!) it must be checked into the source code repository to be available to clients
--https://go.dev/blog/generate

Also fixing a NilClass bug introduced in #6172. The original intent was to check for the match in this unless, so when an error mentioning submodules occurs but didn't match the regex it would raise an error instead of failing with the SharedHelpers exception.