Use exceptions instead of panic

tests/test_cpu_exception.cpp

@@ -0,0 +1,13 @@
+#include "doctest.h"
+#include <cpu/cpu.h>
+#include <memory/ram.h>
+#include <iostream>
+
+TEST_CASE("Undefined instructions throw exception")
+{
+  u8 test_mem[] = { 0xd3, 0x00, 0x00, 0x00 };
+  RAM r(test_mem, 0x4, true);
+  Cpu c(&r);
+
+  REQUIRE_THROWS_AS(c.step(), CpuException);
+}