Merge pull request #332 from VinceMacBuche/bug_4030/None.get

Fixes #4030 : Handle some errors on change request
Merge branch 'branches/rudder/2.4' into branches/rudder/2.6
Merge branch 'branches/rudder/2.6' into branches/rudder/2.7

Conflicts:
	pom.xml
	rudder-core/pom.xml
	rudder-core/src/main/scala/com/normation/rudder/services/modification/DiffService.scala
	rudder-web/pom.xml
Merge branch 'branches/rudder/2.7'
Fixes #3953: Add change request API