"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/033_iferror.zig

@@ -0,0 +1,49 @@
+//
+// Let's revisit the very first error exercise. This time, we're going to
+// look at a special error-handling type of the "if" statement.
+//
+//     if (foo) |value| {
+//
+//         // foo was NOT an error; value is the non-error value of foo
+//
+//     } else |err| {
+//
+//         // foo WAS an error; err is the error value of foo
+//
+//     }
+//
+// We'll take it even further and use a switch statement to handle
+// the error types.
+//
+const MyNumberError = error{
+    TooBig,
+    TooSmall,
+};
+
+const std = @import("std");
+
+pub fn main() void {
+    var nums = [_]u8{ 2, 3, 4, 5, 6 };
+
+    for (nums) |num| {
+        std.debug.print("{}", .{num});
+
+        var n = numberMaybeFail(num);
+        if (n) |value| {
+            std.debug.print("=4. ", .{});
+        } else |err| switch (err) {
+            MyNumberError.TooBig => std.debug.print(">4. ", .{}),
+            // Please add a match for TooSmall here and have it print: "<4. "
+        }
+    }
+
+    std.debug.print("\n", .{});
+}
+
+// This time we'll have numberMaybeFail() return an error union rather
+// than a straight error.
+fn numberMaybeFail(n: u8) MyNumberError!u8 {
+    if (n > 4) return MyNumberError.TooBig;
+    if (n < 4) return MyNumberError.TooSmall;
+    return n;
+}