glfw: README: update usage example

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

README.md

@@ -102,17 +102,29 @@ Now in your code you may import and use GLFW:
 ```zig
 const glfw = @import("glfw");
 
+/// Default GLFW error handling callback
+fn errorCallback(error_code: glfw.Error, description: [:0]const u8) void {
+    std.log.err("glfw: {}: {s}\n", .{ error_code, description });
+}
+
 pub fn main() !void {
-    try glfw.init(.{});
+    glfw.setErrorCallback(errorCallback);
+    if (!glfw.init(.{})) {
+        std.log.err("failed to initialize GLFW: {?s}", .{glfw.getErrorString()});
+        std.process.exit(1);
+    }
     defer glfw.terminate();
 
     // Create our window
-    const window = try glfw.Window.create(640, 480, "Hello, mach-glfw!", null, null, .{});
+    const window = glfw.Window.create(640, 480, "Hello, mach-glfw!", null, null, .{}) orelse {
+        std.log.err("failed to create GLFW window: {?s}", .{glfw.getErrorString()});
+        std.process.exit(1);
+    };
     defer window.destroy();
 
     // Wait for the user to close the window.
     while (!window.shouldClose()) {
-        try glfw.pollEvents();
+        glfw.pollEvents();
     }
 }
 ```