The front end doesn't have to be visual. Something gets the results and emails it. Given what you are describing I wouldn't bother trying to capture other types of errors. Let what process runs your query handle it. Put your try/catch there and you should be good to go. It sounds like it doesn't matter if the connection times out or the database has been deleted, you want to send 'no_data'.

Remember that a union is a single query. Parts of it can't work while other parts fail. It is all or nothing.

select 1

union all

select 1 where 1 = 'x' --this query would fail

Consider the above code. It is a simple query with a union. The second query will cause the whole thing to fail. If you want to capture row counts for each query they need to be run separately. There is no way to know from the results which query in a union returns results.

Perhaps if you can explain the whole process a little more clearly I can see if I can help figure out a way to get what you want.