dbManager should use typed exceptions

[Ext3h]

Right now, dbManager is throwing exceptions of basic type "Exception" to handle empty result sets and every context calling one of these functions is catching all exceptions as well.

This results in also catching actually fatal errors such as MySQL syntax errors and alike. This results in Codeface silently ignoring fatal runtime errors.

dbManager should use a custom exception type for empty result sets instead. All locations expecting exceptions must be changed as well.