Improve UX on invalid expression strings

[bitbrain]

Creating a tree with a BlackboardHasCondition node but the key has an invalid format it causes error spamming:

E 0:00:00:0632   blackboard_has.gd:14 @ tick(): Invalid named index 'destroyed' for base type Object
  <C++ Error>    Condition "p_show_error" is true. Returning: Variant()
  <C++ Source>   core/math/expression.cpp:1507 @ execute()
  <Stack Trace>  blackboard_has.gd:14 @ tick()
                 sequence.gd:23 @ tick()
                 selector.gd:20 @ tick()
                 beehave_tree.gd:130 @ tick()
                 beehave_tree.gd:116 @ _physics_process()

Improve the UX so it is impossible to set up expressions that would never execute.