Make query endpoint optional

[webmaster128]

Closes #699

[ethanfrey]

Looks good.
It would be nice to have a cosmwasm-vm level (integration.rs) test showing the error message when trying to query a contract without the entry point. (Same for migrate).

[webmaster128]

It would be nice to have a cosmwasm-vm level (integration.rs) test showing the error message when trying to query a contract without the entry point. (Same for migrate).

Agreed, will think about how to best integrate this. Not trivial to set up as it would require us to either maintain multiple testing contracts for the VM tests or create artificial test data using WebAssembly text format.

I tested this manually and you get a VmError::ResolveErr as expected. This is tested a bit lower level:

    #[test]
    fn call_function_fails_for_missing_function() {
        let (env, _instance) = make_instance(TESTING_GAS_LIMIT);
        leave_default_data(&env);

        let res = env.call_function("doesnt_exist", &[]);
        match res.unwrap_err() {
            VmError::ResolveErr { msg, .. } => {
                assert_eq!(msg, "Could not get export: Missing export doesnt_exist");
            }
            err => panic!("Unexpected error: {:?}", err),
        }
    }