Don't cleanup clone directory when fail fetch during `clone`

It's a shame to end with a clean directory after (perhaps)
hours of fetching history because a problem during a fetch
cleanup workspace directory even when fetch problems
update release note
Add an help message...
Better management of GitTfsException
Better error reporting
 Better error management when command merge fail
Merge pull request #572 from pmiossec/better_fail_clone_management

Better fail clone management