erp5_base: check consistency when start assignment

   Prevent the user start an inconsistent assignment, since, extra property sheets can include some
   business logic around assignment configuration.
10 jobs for for_testrunner_3 in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

01:03:26

passed ERP5.CodingStyleTest-TestRunner3

02:56:41

failed ERP5.PerformanceTest-Master

00:36:13

failed ERP5.UnitTest-Master

02:24:44

passed ERP5.UnitTest-TestRunner3

06:43:56

passed SlapOS.Eggs.UnitTest-Master.Python2

00:27:11

passed SlapOS.Eggs.UnitTest-Master.Python3

00:24:09

passed Wendelin.UnitTest-Master

00:42:29

passed ERP5.CodingStyleTest-TestRunner3

01:52:06

failed ERP5.UnitTest-TestRunner3

03:20:49