Fixes #3356: cleanly handle ConstraintException on Technique
Merge pull request #11 from fanf/bug_3356/Technique_with_constraint_error_breaks_Rudder

Fixes #3356: cleanly handle ConstraintException on Technique
Merge branch 'branches/rudder/2.4' into branches/rudder/2.5

* branches/rudder/2.4:
  Fixes #3356: cleanly handle ConstraintException on Technique