Replace "Validation Error" with proper user message for theme validation check

[da-real-ivanpan]

Currently, attempt to add new theme with name that is already present in database leads to Validation Error.
This should be improved on, replacing the server error with proper message to user informing them that operation was not successful due to one of the reasons.

Ideally, this feature should not be restricted to theme name uniqueness only: theme passes validation process based on name length, symbols and name uniqueness. The explanation of why transaction failed should pop up to user according to the cause of validation error.