Vague error message for push mirror failure when using gitaly for fetch_remote

What

Instead of a descriptive error the push mirror page is displaying 13:exit status 1.

When manually running the non-gitally version in rails console I get *** Gitlab::Shell::Error Exception: fatal: unable to access 'https://localhost:1943/myrepo/': SSL certificate problem: Invalid certificate chain, which is much more useful.

Why

Returning a sensible error would make it easier for users to diagnose problems

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