golang: Handle errors when fetching default version

If an error occurs when attempting to fetch and parse latest version info, return the latest version in versions list instead.

Fixes SWH-LOADER-CORE-24S

Jenkins job DLDBASE/gitlab-builds #376 succeeded in 1 min 54 sec.
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added 3 commits

• f9aa31dd...d3c5b380 - 2 commits from branch swh/devel:master
• e5e8dce2 - golang: Handle errors when fetching default version

Compare with previous version

Jenkins job DLDBASE/gitlab-builds #384 succeeded in 2 min 0 sec.
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merged