JENKINS-14575: Make failed Git fetch throw IOException

This is a port of a fix for version 1 of the plugin. The previous fix
was in [old-fix] and reviewed at [github]. This version is smaller,
because a failed clone in version 2 throws AbortException instead of
GitException, and it makes sense to keep it that way.

[old-fix] 3ab1db8..f49bc41
[github]  https://github.com/jenkinsci/git-plugin/pull/143