Add Unique index for InternalName and Price system in Price list

[cristinamghita]

Is this a bug or feature request?

What is the current behavior?

Which are the steps to reproduce?

What is the expected or desired behavior?

[metas-dh]

WebUI:

1. when adding a new pricelist with the same price system and internal name as an already existing one,
   new entry is not saved: OK
2. but: no error msg pops up: green bar for indicating entry was saved runs, but when you leave / check another window you get "Do you really want to leave?", indicating the new entry was not really saved: NOK => checked with metas-cg, that's a front end issue, Show error msg when pricelist is not saved bc of UQ index missing for InternalName and Price system in Price list metasfresh-webui-frontend-legacy#2104

Swing:

1. when adding a new pricelist with the same price system and internal name as an already existing one,
   new entry is not saved: OK
2. error msg with info about a pricelist with this internal name already existing: OK

=> pls also add a de_DE msg (e.g. "Eine Preisliste mit diesem internen Namen existiert bereits")
and change the en_US msg ("A pricelist with this internal name already exists")