eliminate OC_Template::printErrorPage in database classes, fixes #12182

as discussed. I replaced all occurences in lib/private/db. 

@MorrisJobke @DeepDiver1975 @LukasReschke 
> no, generic \Exception will end up in 500, HintException in 503

I see - thx - it's not really obvious that HintException will result in 503 - but this is a different discussion :wink: