patch 8.2.3005: Vim9: using a void value does not give a proper error message

Problem:    Vim9: using a void value does not give a proper error message.
Solution:   Give a clear error message. (clodes #8387)
diff --git a/src/vim9.h b/src/vim9.h
index 04972dc..b8c7133 100644
--- a/src/vim9.h
+++ b/src/vim9.h
@@ -91,7 +91,7 @@
     ISN_PCALL,	    // call partial, use isn_arg.pfunc
     ISN_PCALL_END,  // cleanup after ISN_PCALL with cpf_top set
     ISN_RETURN,	    // return, result is on top of stack
-    ISN_RETURN_ZERO, // Push zero, then return
+    ISN_RETURN_VOID, // Push void, then return
     ISN_FUNCREF,    // push a function ref to dfunc isn_arg.funcref
     ISN_NEWFUNC,    // create a global function from a lambda function
     ISN_DEF,	    // list functions