Error Message/Exception Handling Refactoring#4

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SimonFedick wants to merge 2 commits intomasterfrom

exceptionHandleRefactoring

Commits on Oct 19, 2020

SQL Exceptions converted to IllegalArgumentExceptions so that communication hands the information to frontend.
giulianorasper
committed
Refactored DB_UserManager functions to no longer return boolean values and adjusted usages in Conference
SimonFedick
committed