Update invariant ok message (invariants ok)

Signed-off-by: Michael Leuschel <leuschel@cs.uni-duesseldorf.de>

de.prob.ui/src/de/prob/ui/stateview/StateViewStrings.properties

-signalInvariantTooltip=This area signals whether the invariant is violated or not
-signalInvariantGood=invariant ok
+signalInvariantTooltip=This area signals whether the invariants are true or not
+signalInvariantGood=invariants ok
 signalInvariantBad=invariant violated!
 
 signalEventerrorTooltip=This area signals if an error occurred in an event
 signalEventerrorGood=no event errors detected
-signalEventerrorBad=event errors detected!
+signalEventerrorBad=event error(s) detected!
 
 signalModelmodifiedTooltip=This area signals whether the model has been changed. Doubleclick to reload it
 signalModelmodifiedBad=The model was modified!