Add more expression test cases, fix bugs

Also added a few more comments + reorganized exceptions that are
invoke-internal.

(cherry picked from commit cbd686548fd62248fabbaab551a6875a14170957)

worldedit-core/src/main/java/com/sk89q/worldedit/internal/expression/invoke/CompilingVisitor.java

@@ -678,13 +678,10 @@ class CompilingVisitor extends ExpressionBaseVisitor<MethodHandle> {
         // MH:oldResult,result = (ExecutionData)Double
 
         // Execute `oldResult` but ignore its return value, then execute result and return that.
-        // If either result is `defaultResult`, it's bogus, so just skip it
+        // If `oldResult` (the old value) is `defaultResult`, it's bogus, so just skip it
         if (oldResult == DEFAULT_RESULT) {
             return result;
         }
-        if (result == DEFAULT_RESULT) {
-            return oldResult;
-        }
         // Add a dummy Double parameter to the end
         // MH:dummyDouble = (ExecutionData, Double)Double
         MethodHandle dummyDouble = MethodHandles.dropArguments(

worldedit-core/src/test/java/com/sk89q/worldedit/internal/expression/ExpressionTest.java

@@ -55,7 +55,17 @@ class ExpressionTest extends BaseExpressionTest {
             testCase("0 || 5", 5),
             testCase("2 || 5", 2),
             testCase("2 && 5", 5),
-            testCase("5 && 0", 0)
+            testCase("5 && 0", 0),
+            // check ternaries
+            testCase("false ? 1 : 2", 2),
+            testCase("true ? 1 : 2", 1),
+            // - ternary binds inside
+            testCase("true ? true ? 1 : 2 : 3", 1),
+            testCase("true ? false ? 1 : 2 : 3", 2),
+            testCase("false ? true ? 1 : 2 : 3", 3),
+            testCase("false ? false ? 1 : 2 : 3", 3),
+            // check return
+            testCase("return 1; 0", 1)
         );
         return testCases.stream()
             .map(testCase -> DynamicTest.dynamicTest(