Fix error reporting when signature of bound function is not void

[elverskog]

Took a bit to figure out an error related to a bound (win.binding) function, I created, not returning void.
I was trying to return !void, to account for a try in the function.
With the current version I got an error summarized here:

cannot format optional without a specifier (i.e. {?} or {any})
@compileError("cannot format optional without a specifier (i.e. {?} or {any})");
.../src/webui.zig:777:46: note: called from here const err_msg = std.fmt.comptimePrint(

This patch (really created by https://github.com/elpekenin) tries to offer a clearer response, as oppose to an error in the lib.

[jinzhongjia]

Great PR, thank you for your work