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

as discussed. I replaced all occurences in lib/private/db. 

@MorrisJobke @DeepDiver1975 @LukasReschke 
I'm not too sure about this one - HintException will translate to http status code 500 - not 503.
The header() calls will be overwritten later.

We should add an httpstatus code to our exception classes - defaulting to 500