better error handling

src/main.zig

@@ -51,6 +51,13 @@ fn handleMemoryCommand(memory: *mem.Memory, it: anytype) !void {
                 const value = try std.fmt.parseUnsigned(u8, maybe_value.?, 10);
                 _ = try memory.writeByte(addr, value);
             }
+        } else {
+            std.debug.print(
+                \\Invalid argument '{s}'. Usage:
+                \\ mem read <addr>
+                \\ mem write <adrr> <value>
+                \\
+            , .{next_argument});
         }
     }
 }