"999 is enough for anybody" triple-zero padding (#18)

When I hit 999 exercises, I will finally have reached the ultimate
state of soteriological release and no more exercises will be needed.
The cycle will be complete. All that will be left is perfect quietude,
freedom, and highest happiness.

exercises/023_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;
+    }
+}