Add error printing on invalid codegen

[clickingbuttons]

Useful for when Zig breaks codegenned syntax and for codegen devs. Here's one way to do it:

fn format(allocator: Allocator, fname: []const u8, code: [:0]const u8) ![]const u8 {
    var ast = try std.zig.Ast.parse(allocator, code, .zig);
    defer ast.deinit(allocator);

    if (ast.errors.len > 0) {
        for (ast.errors) |err| {
            var buf = std.ArrayList(u8).init(allocator);
            defer buf.deinit();
            ast.renderError(err, buf.writer()) catch {};
            log.err("formatting {s}: {s}", .{ fname, buf.items });
        }
        return code;
    }

    return try ast.render(allocator);
}

[travisstaloch]

partially addressed in 0a7d9d4

i don't consider this complete as the error message doesn't show any of the generated code which caused the error. as-is it only shows the parse error messages.