Change error for unknown union serialization (#511)

Author: hpmellema

codegen/core/src/it/java/software/amazon/smithy/java/codegen/test/UnionTest.java

@@ -26,7 +26,6 @@
 import software.amazon.smithy.java.codegen.test.model.NestedUnion;
 import software.amazon.smithy.java.codegen.test.model.UnionType;
 import software.amazon.smithy.java.core.schema.SerializableShape;
-import software.amazon.smithy.java.core.serde.SerializationException;
 import software.amazon.smithy.java.core.serde.ShapeSerializer;
 import software.amazon.smithy.java.core.serde.document.Document;
 import software.amazon.smithy.model.shapes.ShapeType;
@@ -112,6 +111,6 @@ void unknownUnionDeser() {
     @Test
     void unknownUnionSerFails() {
         var union = UnionType.builder().$unknownMember("foo").build();
-        assertThrows(SerializationException.class, () -> Document.of(union));
+        assertThrows(UnsupportedOperationException.class, () -> Document.of(union));
     }
 }

codegen/core/src/main/java/software/amazon/smithy/java/codegen/generators/UnionGenerator.java

@@ -20,7 +20,6 @@
 import software.amazon.smithy.java.core.schema.Schema;
 import software.amazon.smithy.java.core.schema.SchemaUtils;
 import software.amazon.smithy.java.core.schema.SerializableStruct;
-import software.amazon.smithy.java.core.serde.SerializationException;
 import software.amazon.smithy.java.core.serde.ShapeSerializer;
 import software.amazon.smithy.java.core.serde.document.Document;
 import software.amazon.smithy.model.Model;
@@ -217,7 +216,7 @@ public static final class $$UnknownMember extends ${shape:T} {
 
                     @Override
                     public void serialize(${shapeSerializer:T} serializer) {
-                        throw new ${serdeException:T}("Cannot serialize union with unknown member " + this.memberName);
+                        throw new ${unsupportedOperation:T}("Cannot serialize union with unknown member " + this.memberName);
                     }
 
                     @Override
@@ -230,7 +229,7 @@ public <T> T getValue() {
                     }
                 }
                 """;
-            writer.putContext("serdeException", SerializationException.class);
+            writer.putContext("unsupportedOperation", UnsupportedOperationException.class);
             writer.putContext("string", String.class);
             writer.write(template);
             writer.popState();