Allow blocking flows to return messages displayed to the user

Summary Currently, when a blocking flow is ended on the unhappy path, the only message that user sees is 'An unexpected error occured'. However it would be nice to let flow return a meaningful message that will be displayed to the user. New exception class shoould be added that will allow error message to propagate to the user level. Basic Example _No response_ Motivation For example, we created a flow to enforce certain user permissions. E.g. some combination of field values is not allowed. We can block such combinations with a flow, but we cannot inform the user why the record cannot be updated. We would like instead show user a message 'This combination is not allowed'. Detailed Design [This piece of code]( always return 'Unexpected error' when user is not admin. Add new exception class that will allow error message to propagate down to the user level. Something like that in script block: The user should see the message 'Condition…