Introducing @RC builtin variable

We've long needed a way to see the most recent SQLite result code SQLite in the context of say a catch block (most other times you can assume SQLITE_OK was the last result code otherwise control flow would transfer elsewhere. Sometimes SQLITE_ROW or SQLITE_DONE might be the current result code.

Soon we'll provide a sample header that declares the most common error codes in an enum but for now you could do something like this:

-- pasted from the sqlite.c
#define SQLITE_BUSY         5   /* The database file is locked */

-- this is a contrived example
create proc get_first_foo(out can_retry bool not null)
begin

  -- can_retry is set to 0 automatically, language semantics guarantee this

  begin try
    select foo from bar limit 1;
  end try;
  begin catch
    set can_retry := (@rc == SQLITE_BUSY);
    throw; -- rethrow the original error
  end catch;
end;