Do not report version when upstream check fails

[stoned]

... otherwise you could get something like the following (here sops is run without network connection available):

$ ./sops --version
sops 3.7.3
[warning] failed to retrieve latest version from upstream: Get "https://raw.githubusercontent.com/mozilla/sops/master/version/version.go": dial tcp: lookup raw.githubusercontent.com: Temporary failure in name resolution

[warning] failed to compare current version with latest: Version string empty
 (latest)
$

[hiddeco]

Already fixed via #1115

[felixfontein]

@hiddeco this hasn't been fixed. What this PR is fixing is that in case of a comparison error (when the new version is fetched), that both the error and (latest) (which is wrong) are printed.

(Probably the whole comparison logic should be skipped when a network error happens, though, instead of a network error triggering the "Version string empty" error.)

[hiddeco]

@felixfontein you are absolutely right, thanks.

@stoned any chance you can rebase and sign-off your commit?

[hiddeco]

Thank you @stoned 🙇 🥭