fix: Return fatal error in operations

Adjust all operations to return a fatal error instead of returning null. This is required to limit the access to the special input symbol `?` outside an unary-tests.

Restructure some helper methods to restrict the handling of fatal errors to a few places. This should help to reduce possible problems with fatal errors.

Move some methods to group different aspects together.

src/main/scala/org/camunda/feel/impl/builtin/BuiltinFunction.scala

@@ -17,7 +17,7 @@
 package org.camunda.feel.impl.builtin
 
 import org.camunda.feel.logger
-import org.camunda.feel.syntaxtree.{Val, ValError, ValFunction, ValNull}
+import org.camunda.feel.syntaxtree.{Val, ValError, ValFatalError, ValFunction, ValNull}
 
 object BuiltinFunction {
 
@@ -34,9 +34,10 @@ object BuiltinFunction {
   }
 
   private def error: PartialFunction[List[Val], Any] = {
-    case args if (args.exists(_.isInstanceOf[ValError])) =>
-      args.filter(_.isInstanceOf[ValError]).head.asInstanceOf[ValError]
-    case args                                            =>
+    case args if args.exists(_.isInstanceOf[ValFatalError]) =>
+      args.find(_.isInstanceOf[ValFatalError])
+    case args if args.exists(_.isInstanceOf[ValError])      => args.find(_.isInstanceOf[ValError])
+    case args                                               =>
       val argumentList = args.map("'" + _ + "'").mkString(", ")
       ValError(s"Illegal arguments: $argumentList")
   }