Properly report errors

src/main.zig

@@ -39,7 +39,17 @@ pub fn main(init: std.process.Init) !void {
     var file_reader = file.reader(io, &file_buf);
     const reader = &file_reader.interface;
 
-    var class = try Class.parse(reader, allocator);
+    var class = Class.parse(reader, allocator) catch |err| switch (err) {
+        error.InvalidMagicNumber => {
+            try stderr.writeAll("Invalid magic number.\n");
+            return;
+        },
+        error.InvalidTag => {
+            try stderr.writeAll("Unrecognized tag type.\n");
+            return;
+        },
+        else => return err,
+    };
     defer class.deinit();
 
     try stdout.print("{f}", .{ class });