Re-enable checking of gauge availability This partly reverts rev. 1cb6676d1510, which removed this code in favour of handling database errors later. The thinko in this was that possibly many NtS messages for the same gauge would lead to many errors for a single reason and the same amount of unnecessary database round-trips. Checking whether the current user is allowed to import data for a gauge now also handles sys_admin correctly and the import stops with status unchanged if no such gauge is available.