[JENKINS-23641] - Handle possible nulls if Git checkout fails

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
[JENKINS-23641] - Direct tests for the issue

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
[FIXED JENKINS-25312] - Build status on update failure is configurable

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Merge pull request #46 from oleg-nenashev/JENKINS-23641

[JENKINS-23641,JENKINS-25312] - Handle possible nulls if Git checkout fails