Rudder

When a new system technique is missing, we get a useless error message:

[2021-11-30 13:32:11+0100] INFO  migration - Check if all Rudder 7.0 system technique are correctly loaded in technique library
[2021-11-30 13:32:13+0100] INFO  git-repository - git-gc performed on configuration-repository in PT5.763S
[2021-11-30 13:32:13+0100] ERROR migration - Error during migration of policy server system configuration to Rudder V7.0 data. It is likely that Rudder won't work as expected. Please check the log above. Error was: SystemError: An error occured; cause was: java.nio.file.NoSuchFileException: /var/rudder/configuration-repository/techniques/system/rudder-service-postgresql/1.0/placeholder
 ->

The code was thought to produce:

s"New system technique '${t}' is not available in technique repository. " +
 s"It is necessary to be able to migrate old system configurations to 7.0." 

but that code is not reached.