one error message for one or more fields

Hello,

I am creating a forum where volunteers can register. One aspect of it is choosing the time they are available to help. There are many options to choose and I have broken them into 12 groups using a drop down list so it is easier to manage and read.

No select boxes are mandatory; however, I want to determine if at least one option has been selected. Therefore, I am trying to create my own custom validation to determine this.

You can view the form here.

I am using contact form 7 version 4.8.

Thanks in advance.