Throw GitLockFailedException on repository lock failure

Refs #17, #31, jenkinsci/git-plugin#177
This modification tries to throw GitLockFailedException when a concurrent build has a lock file on the repository, and should reflect @ndeloof 's [initial suggestion](https://github.com/jenkinsci/git-client-plugin/pull/17#commitcomment-3888726), which I could not get it at a first glance.

Sorry to bother you repeatedly. I'd happy to make modifications if problem still exists.
Thanks!