Make sure unsupported/illegal instructions are caught

src/core.rs

@@ -47,6 +47,8 @@ impl Core {
                 }
             };
 
+            assert_eq!(instr & 3, 3, "Compressed instructions not supported");
+
             let instr = Instruction(instr);
 
             let runner = find_runner(instr);
@@ -65,6 +67,9 @@ impl Core {
                         break;
                     }
                 }
+            } else {
+                eprintln!("Invalid Instruction");
+                break;
             }
         }
     }