Submissions after deadline show confusing errors

[zerothabhishek]

For example, If the CFP closes at 12 midnight, and we draft a proposal at 11:55, and submit it at 12:02, we get the following error:

Select a valid choice, 17 is not one of the available choices.

Near the Proposal Type field.

The error message should be more helpful, something like the deadline has passed.

[dishantsethi]

Hey, I want to work on this issue. How can I test this on localhost? @zerothabhishek