Added ex 21-26 for error handling

23_errors3.zig

@@ -0,0 +1,28 @@
+//
+// One way to deal with error unions is to "catch" any error and
+// replace it with a default value.
+//
+//     foo = canFail() catch 6;
+//
+// If canFail() fails, foo will equal 6.
+//
+const std = @import("std");
+
+const MyNumberError = error{ TooSmall };
+
+pub fn main() void {
+    var a: u32 = addTwenty(44) catch 22;
+    var b: u32 = addTwenty(4)  ???   22;
+
+    std.debug.print("a={}, b={}", .{a,b});
+}
+
+// Please provide the return type from this function.
+// Hint: it'll be an error union.
+fn addTwenty(n: u32) ??? {
+    if (n < 5) {
+        return MyNumberError.TooSmall;
+    } else {
+        return n + 20;
+    }
+}