Display a better message when the ExternalDataSources name isn't unique

Currently when the ExternalDataSources name isn't unique the backend sends a response like so

"org.springframework.context.NoSuchMessageException: No message found under code 'unique'

There should actually be a better message than that. It needs adding to the message bundle in the backend. Its a missing translation key.

Update: Message should be the same as the toast message defined in ERM-437:

"External data source was not saved. Name must be unique. A data source with the name "dummy3" already exists."