Better error message when trying to get Beam Schema from proto schema…

… with Struct (apache#32260)

* Better error message when trying to get Beam Schema from proto schema with Struct

* minor fix - consolidate

sdks/java/extensions/protobuf/src/main/java/org/apache/beam/sdk/extensions/protobuf/ProtoSchemaTranslator.java

@@ -169,9 +169,12 @@ static synchronized Schema getSchema(Descriptors.Descriptor descriptor) {
     if (alreadyVisitedSchemas.containsKey(descriptor)) {
       @Nullable Schema existingSchema = alreadyVisitedSchemas.get(descriptor);
       if (existingSchema == null) {
+        String name = descriptor.getFullName();
+        if ("google.protobuf.Struct".equals(name)) {
+          throw new UnsupportedOperationException("Infer schema of Struct type is not supported.");
+        }
         throw new IllegalArgumentException(
-            "Cannot infer schema with a circular reference. Proto Field: "
-                + descriptor.getFullName());
+            "Cannot infer schema with a circular reference. Proto Field: " + name);
       }
       return existingSchema;
     }
@@ -339,7 +342,7 @@ private static FieldType beamFieldTypeFromSingularProtoField(
             fieldType = FieldType.logicalType(new NanosDuration());
             break;
           case "google.protobuf.Any":
-            throw new RuntimeException("Any not yet supported");
+            throw new UnsupportedOperationException("Any not yet supported");
           default:
             fieldType = FieldType.row(getSchema(protoFieldDescriptor.getMessageType()));
         }