Improve the debug messages when invalid instructions are found (again)

src/core.rs

@@ -78,7 +78,7 @@ impl Core {
                     }
                 }
             } else {
-                eprintln!("Invalid Instruction {:08x}", instr.0);
+                eprintln!("Invalid Instruction {:08x} at PC: {:x}", instr.0, self.pc);
                 break;
             }
         }